Turning Invisible should not make a character automatically Hidden

There's a couple reasons why.

What did they roll for Stealth?

If a character is "automatically hidden", then as DM, you need to come up with a way to adjudicate any potential attempts an opposing character might make to detect their presence. You can't just have them roll a D20, because if they roll poorly, then they'd be automatically detected by the passive perception of their enemies, and wouldn't be "Automatically hidden"; that's a paradox.

And you can't necessarily just take their passive Stealth score either, since that might not be high enough to pass the passive Perception score of their enemy. And if you base it on the perception scores of their enemy, then it becomes a weird mechanical consequence where the better a creature's perception is, the better Invisibility becomes against them, which cannot be right.

It makes Stealth too easy

Assuming you resolve the previous issue, this change also makes invisibility too powerful in a combat scenario.

Under the normal rules:

• A Wizard casts Invisibility on themselves


• They then run away their maximum movement speed


• No Opportunity Attacks are triggered because Opportunity attacks require the reacting creature to be able to see their target


• The creature (and maybe some of their allies) immediately run to the Wizard's new location and attack them (with disadvantage, because unseen)


• The Wizard may or may not lose concentration if some of the attacks land


• If they kept the spell up, they take the Hide action, rolling Stealth, and then continuing to run away


• If the Stealth Score beats the enemy creatures' Passive Perception scores, they begin using the Search action to try to find the Wizard. If it doesn't, the last few steps happen again.


• Once the Wizard finally beats the Passive Perceptions of their enemies, they Search like before.


• On success, they run up to the Wizard but do not make any attacks


• On failure, they simply cannot find the Wizard

Conversely, under this rule:

• A Wizard casts Invisibility on themselves


• They run away at max speed


• No Opportunity Attacks


• The enemy creatures take the Search action (they cannot possibly succeed on their Passive Perception checks)


• On success, they run up to the Wizard but make no attacks


• On failure, they are unable to find the Wizard

Two of the most critical chances the enemy creatures had to take down the Wizard has been removed. Under the normal rules, an Invisible Wizard can be thwarted by attacks made against them, and a failed Stealth check.

Under your modified rules, neither of those are credible threats. The only thing an enemy creature has is to either succeed at their Perception checks (probably more than once before it works!) or target a random spot and hope they land a hit. That is a very significant increase in power.

"Okay, so what if turning invisible instead means they make an automatic /attempt/ to hide?"

This is a lot less overpowered, but still potentially problematic. It represents a middle ground between the normal rules and the scenario with your rule:

• A Wizard casts Invisibility on themselves


• They then run away their maximum movement speed


• No Opportunity Attacks are triggered because Opportunity attacks require the reacting creature to be able to see their target


• The Wizard gets an automatic attempt to Hide, rolling Stealth.


• If the Stealth Score beats the enemy creatures' Passive Perception scores, they begin using the Search action to try to find the Wizard. If it doesn't, they run up and make their (with disadvantage) attacks, Concentration, Hide Action, blah blah blah.


• Once the Wizard finally beats the Passive Perceptions of their enemies, they Search like before.


• On success, they run up to the Wizard but do not make any attacks


• On failure, they simply cannot find the Wizard

So it's less extreme, but it still means a lowered chance of successfully shutting down the retreating Wizard. In particular, it means that the enemy creatures probably won't get to make any attacks against them, and therefore no attempts to break concentration. That still makes Invisibility a lot more powerful.

This also steps on the toes of the Rogue, especially Arcane Trickster, class

The advantage that Rogues (and certain other classes like Ranger) gain is the ability to, among other things, take the Hide action as a Bonus Action in addition to a regular Action. So if an Arcane Trickster were to cast Invisibility during their turn, they'd then be easily able to Hide afterwards, doing as part of the features granted to them by their class/archetype something that you're now going to give for free to everyone.

Conversely, this could also make Arcane Tricksters very powerful: if they no longer need to use the Hide [Bonus-]Action, then they could do something else, like Dash or do something with their Mage Hand Legerdemain feature. No longer requiring an action to use their Stealth would free up their Action Economy significantly, and for a Rogue, that can be a dangerous thing to do.

Conclusion

The stealth rules are written the way they are for a reason, and if you're planning to change those rules, you need to have a clear conception of why you're changing them and what the consequences of that change will be. The rules for Hiding (PHB, pg. 177) specifically call out Invisibility as a condition under which hiding is "always allowed", which implies as a consequence that being Invisible does not make a creature automatically hidden:

You can't hide from a creature that can see you clearly, and if you make noise (such as shouting a warning or knocking over a vase), you give away your position. An invisible creature can't be seen, so it can always try to hide. Signs of its passage might still be noticed, however, and it still has to stay quiet.

—Hiding, PHB, pg. 177

So in general, I would not advise this change. Don't turn Invisibility into an automatic "Get out of Combat Free" card. Specializing in Stealth will already make most Invisibility uses very powerful; it's not necessary to go any further.

The rule you suggest is a problem, but probably isn't necessary.

I disagree with the quoted text -- that the DM "shouldn't" rule you automatically hidden and that it's always "horribly unbalanced" -- but the other option isn't "invisible creatures are always automatically hidden", and that rule would almost certainly be a problem, and may be incoherent as it's written.

First, we should talk about what the actual hiding rules are.

Based on the sidebar on p177 of the Player's Handbook, normally you take an action to Hide, and roll your Stealth check. That sets the difficulty for any other creature to find you. Nothing in there says the creatures around you automatically roll perception against your stealth to determine if you became hidden; you become hidden immediately and set the difficulty for searchers to locate you.

Now, if your roll is lower than the passive perception of some of the creatures around you, you never really become hidden to them -- they immediately see you without taking an action. But you could certainly become hidden to some creatures in a fight and not to others, and there isn't really a chance of you using your action to hide and some goblin immediately rolling a natural 20 and causing your hide to fail when you would have beaten its passive perception. The goblin would need to spend its action to attempt to find you using your previous Stealth check as a DC.

Invisibility only makes you able to hide, but it doesn't inherently hide you, and it doesn't change any of the above rules about hiding.

So if you say "automatically hidden", I think what you really mean is "you automatically take the Hide action", which includes a Stealth roll to set your DC. Otherwise, you're saying you become "hidden" but without a number to say how well you're hidden and whether a Dire Wolf's passive perception is good enough to know where you are despite the spell.

So in play...

If a guy in a tense melee suddenly goes invisible, by the rules, he can't take the Hide action right away (usually) because he just used his action to cast a spell or use an ability to become invisible. And that's fine; it makes a lot of sense that if somebody goes invisible in the middle of a fight, everyone still has a pretty solid idea of where they were a second ago and where they might have gone.

Giving that guy an auto-hide effect would make everyone around him immediately unaware of his presence, which is kind of crazy. It suggests everyone around immediately has amnesia even if the newly-invisible guy doesn't even move away or do anything to confuse the question of where he is now.

By contrast, if a guy who's been shooting a bow or casting spells from 60+ feet away suddenly goes invisible, it's equally absurd to suggest that everyone knows precisely where they are afterward, and doubly so if they did something to break the line of sight first, like going around a corner or into a fog cloud.

So I would argue that your proposed rule makes invisibility rather too good in close combat, but the quoted text has some strange and unintended effects.

I think that's why it's left up to the DM. If you can use your turn to take the Hide action, then yeah, you're definitely hidden, no arguments (assuming you surpassed everyone's passive perception). If even you don't have time to actively hide, sometimes you still should count as hidden, but not always. Sometimes the disadvantage on attacks against you is enough to account for "it isn't totally obvious where you are".

In general, my rule of thumb for both creatures and objects would be:

• If you could see (or otherwise sense) the thing right before it turned invisible and are near it, you know where it is until it takes an action to hide (if it's a creature).




• If you could see the thing as it turned invisible but it was a moderately long distance away, you know about where it was but don't know its precise current location, so it counts as hidden.




• If you couldn't see the thing when it went invisible, then it's automatically hidden from you.

All of those are subject to the usual Hiding rules relating to what you can do without giving yourself away, naturally, but when you stop being noisy, it's like you just then went invisible again. Shouting while invisible is a dead giveaway, but if you shout from 100 feet away, it's unreasonable to assume everyone can pinpoint your location from that.

We've tried this, and it's not as bad as you'd theorize.

My tables have sometimes tried this type of mechanic. The thought behind doing it was that it made 'logical' sense. I'm not sure I agree with that, but it is what it is. The idea that being invisible wasn't as cool as the rules made it out to be I think was more of the driving factor.

The good

When a PC did it

Being invisible felt really fun. Enemies didn't know your location and it 'felt' more fantasy.

When an NPC did it

This increased the combat tension. Not knowing where enemies were ratcheted up the seriousness of the event. It was frustrating not knowing, but it also made the encounter very serious.

The bad

The bad is that things that normally should work with the rules...don't. There are a lot of mechanics that still work if you know location even if you can't see them. This breaks those completely. It creates limitations where there weren't any.

In the end, it's playstyle

Honestly, I don't think it was a major issue other than having to rethink and adjudicate how things worked. It did add a lot to the combats to make them much harder and interesting, but some of the frustrations were real as well.

I don't think it's the end of the world when we did it, but I've decided to stick to the RAW rules generally to make adjudication easier. However, if my table was on board with it, I'd likely be willing to go back to the houserule.

You have essentially removed stealth (and by that extension rogues) from the game

Invisibility is a 2nd level spell, and it lasts for an hour for each casting. So at 3rd level, your wizard can be completely undetectable for 2 hours a day. Comparatively, your min-maxed rogue (20 dex) will have a stealth score that's +8 (+4 from Dex, +4 from expertise proficiency). The lowest passive perception you're likely to run up against is a 10, meaning that your rogue has a 5% chance to fail every stealth roll they make. This may not seem like much, but it quickly adds up. The chance of an absolutely min-maxed level 3 rogue of sneaking by a passive-perception 10 (lowest reasonable) is:

• 1 Check = 95%


• 2 Checks = 90%


• 3 Checks = 86%


• 4 Checks = 81%


• 5 Checks = 77%

So if the rogue needs to sneak past 5 guards to steal something, and the rogue has spent every single ability point to get their stealth bonus as high as possible, and the guards all have the lowest passive perception possible, the rogue has a 23% chance of failing. And that's just assuming you only ever need to beat passive perceptions. If there are any active perception rolls, the rogue's chance of success drops even further.

Your wizard, however, has a 0% chance of failing. The wizard can also sneak past 6 guards, or 10 guards, or 5,000 guards, all with that same 0% chance of failure. The rogue can't sneak in well lit rooms, the wizard doesn't care. The rogue can't go near torches, the wizard can. The rogue can't walk over dust or ash or anything else that would leave footprints, the wizard doesn't have to worry about that. And for all her trouble, the rogue STILL has a 23% chance of total failure, getting caught, alerting the garrison, having the item chucked into a safe, etc.

At this point, being a stealthy rogue has no benefits. Your one thing you're good at, stealth, is completely worthless. In fact it's worse than worthless; you've wasted an Expertise slot training a skill that essentially doesn't exist. Why would anyone ever stealth anywhere when invisibility is perfect? If a rogue gets caught when they're stealthing, that's it, they're spotted, and they can't even begin to hide again until they're completely obscured from the people chasing them. A wizard on the other hand can just run around the corner and now they're perfectly hidden again (unless you created a new system for hiding after being discovered). There isn't even a good reason for rogues to take Arcane Trickster for invisibility, because your stealth score gives you nothing. Being invisible is equivalent to having an infinite stealth score, so why would you pump points into making yours an 8 vs a 4?

Skills in 5e are weak. They fail a lot, and it's generally impossible to be anything other than 'decent but not great' at things. Spells are generally balanced around this, which is why invisibility is generally useless unless you also have a high stealth score. Removing this restriction will make stealth completely irrelevant, and remove the entire rogue archetype. Essentially with this change, if your group is planning a heist, the most helpful thing the rogue can do is sit quietly in the tavern while the spellcasters go and do the thieving.

My biggest problem with this homebrew is it seems like it makes things unnecessarily inconsistent, especially if you're guaranteeing that they are hidden rather than making them roll to see how well hidden they are.

The rules already require you to be unseen to attempt to hide. This homebrew is essentially saying "not being visible because of invisibility is more special than not being visible because you're behind a wall", even though someone behind a wall is just as unobservable to you as an invisible person behind a wall or an invisible person in the open.

To highlight some of the weird cases this produces:

Casting invisibility makes your position more undetectable than running behind a large obstacle to an observer on the other side- in both cases, you've left their vision, but the latter situation has to take an additional hide action for you to truly lose their position because anyone going invisible "is going to be quiet", even though hiding was also the purpose of moving to the other side of the obstacle- they'd be being quiet once they're on the other side as well, but it still takes the Hide action to do so.

In fact, once I am behind the obstacle, it is better for me to go invisible than to take the Hide action against someone trying to keep track of my position, even if they do not intend to use sight to do so- it's a perfectly valid method in combat to use an active perception check to determine the location of a creature hiding from you that you cannot see, like a sneaky rogue on the other side of the wall from you trying to move through the adjacent room and come around from behind. There's no requirement that you try to get them into your line of sight to do so.

Your list of assumptions makes it so that, even though I can't see them in either the visible-but-hiding or invisible case, I'm seemingly never allowed to detect the latter even though to me they're equally not visible.

Doesn't Make Sense Based on What the Spell Does

Beyond balance issues, what you are asking about is probably superfluous given that the creature is already automatically unseen. I think we have to look at the nature of being hidden and the nature of being unseen before we can explore balance.

Hidden and Unseen are Not the Same Thing

Being hidden is the state of being unseen and unheard as per RAW. Presumably it also involves being on some level untouched, untasted, and unsmelled (though the classic "fee, fie, foe, fum; I smell the blood of an Englishman" scenario may also make sense as still being hidden, so long as you are otherwise undetectable). If your not playing strictly RAW this is very much a DM call.

Invisibility automatically makes one unseen by anyone without a "see invisibility" ability. It wouldn't logically make them unheard or un-anything-else. It would logically make it much easier for them to hide (advantage?) but that is still a different thing.

Hidden, Unseen, and Otherwise Undetected are All Relational

When you are hidden or when you are in some specific sensory way undetected, this is in relationship to one or more creatures, not necessarily everybody. Effectively automatic hiding may make sense under some conditions in relation to creatures that rely primarily or entirely on sight, but a dog, for example, would probably still be able to know almost exactly where you are based on smell, though it might be two confused to make sense of the situation. A band of new players I DMed for were very disappointed when their first experiment using an invisibility spell involved sneaking through a kennel.

You also may also be visually semi-detectable based on object interactions, including things like footprints or walking through smoke. And while mundanely unseen by one creature, many others may have a clear line of sight on you.

Being Unseen is Mechanically Very Powerful on Its Own

As per RAW, you have advantage in attacks against a creature who can't see you. A creature has disadvantage attacking a creature they can't see. This is much of what one gets out of more generally being hidden in combat. It will not, of course, keep creatures from attempting to investigate, attack, or pursue you, but it is hard to see a reason why it should, and will likely make it much more difficult for them.

The Proposed Rule Substantially Undermines Class Abilities, the Hide Action, and a Skill for No Justifiable Reason

Both Rogues and Rangers have abilities to hide as a bonus action (in the case of a Ranger, a 14th level ability). It undermines these skills to bestow being hidden on invisibility. Stealth is a skill that characters choose among there limited selection of skill proficiencies, and one that invisibility already undermines the value of. Why should it further undermine this value in relation to aspects of sensory perception which have nothing to do with visibility? Finally when other characters have to burn an action for a mere attempt to hide, it seems unbalanced for an already arguably very powerful spell to also take and auto-accomplish this action for free.

In conclusion, the person going invisible is already giving enemies, even ones who know exactly where they are disadvantage, which is already powerful, and, given the larger implications and requirements of being hidden, it is unbalanced to make them completely hidden without stronger justifications for how this would lead to the enemy not knowing where they are despite the enemies other senses and reasoning skills.

Were you to create a spell of general undetectability at an appropriately balanced level that would be a very different matter.