How should I implement False Appearance?

Question

In the MM, several creatures have the ability False Appearance, which is described with the general structure (emphasis mine)

While [the creature] remains motionless, it is indistinguishable from a normal [ostensibly harmless object].

"Indistinguishable" is a very strong term and makes it seem like this ability imparts nearly flawless camouflage. It looks, smells, and even thinks like what it's supposed to be. That obviously can't be right, but how far do I dial it back?

Now, I will be downright gleeful if anyone can provide RAW limitations to this ability. However, assuming that isn't possible, I would appreciate guidance on how to implement this ability in a way that is fair to the players and in keeping with the ability's intent. In particular:

1. Does an ordinary, completely harmless stalactite gain Unseen Attacker advantage when it attempts to throttle a character within its reach?
2. Is it only visually indistinguishable, or should this also fool other senses?
3. Should the PCs even get a chance to use Perception (either passive or as a check)?
4. Once it's moved, should the stalactite need an action to become motionless again?
5. Should indistinguishability extend to magic (e.g. Detect Magic, Truesight)?

I should have probably asked this before designing the Darkmantle maze for my next session...

Answer 1

As far as I can determine, there are no guidelines for exactly what this ability covers. However, I would assume that this is referring to physical indistinguishability (is that a word?) and therefore I would rule as the following:

1. Does an ordinary, completely harmless stalactite gain Unseen Attacker advantage when it attempts to throttle a character within its reach?

Yes. If a creature is unseen when it attacks, it has the advantage. Note that attacking will reveal it, so this will only apply to the first attack the creature makes.

2. Is it only visually indistinguishable, or should this also fool other senses?

This is tricky. On the other hand, it has to remain still for this ability to function, so sound is not an issue. I wouldn't assume the creature would have strong BO - this would invalidate the ability somewhat. However, if a player went up to it and touched it or sniffed at it, I would at the very least allow them to make a Perception or Investigation roll.

3. Should the PCs even get a chance to use Perception (either passive or as a check)?

As I mentioned in the previous point, if a player was to feel the creature or sniff at it from right up close, I'd probably allow Perception to apply. But if this ability was meant to interact with Perception, I'd assume it would say something like "While [the creature] remains motionless, it is indistinguishable from a normal [ostensibly harmless object] and gains a +X bonus to its Stealth check." Since it doesn't include anything to make it more difficult to succeed on a Perception check, my assumption is that it's not meant to be on the receiving end of Perception checks at all.

4. Once it's moved, should the stalactite need an action to become motionless again?

I would rule that the creature would have to make a Hide action to allow this ability to function. Note that this shouldn't really matter - if it's not in the vicinity of the players, it won't matter whether it needs an action or not. If it is in the vicinity of the players, I'd imagine they'd notice a stalactite shuffling along the ceiling and pay attention to where it stopped moving.

5. Should indistinguishability extend to magic (e.g. Detect Magic, Truesight)?

I would assume not. However, Detect Magic would only detect a creature with a magical effect on it, and this ability isn't an illusion that can be seen through with Truesight, it's a purely physical effect. Something like Detect Thoughts, on the other hand, should work fine.

Answer 2

Looking through the list of creatures with False Appearance, they will look exactly like their lookalike form. No perception passive or active to attempt to detect anything unusual about the creature.

Most would smell almost exactly like their base look alike form or if they smell slightly different it would be almost un-noticible to the point where the players would have to be actively sniffing the creature in question and by then I'd say it's pretty much irrelevant.

Touch once again, they'll feel almost identical but that is more due to the nature of their physical makeup than false appearance itself.

Any sense besides sight would require the player to get close enough to the creature that it would attack or do something else that might give away it's true nature.

As for magic, anything based on vision such as true seeing would not work. Something that senses motive or thought or something based on other senses has a chance to pick up the creature. The only creature I see that might be picked up by detect magic would be the Animated Armors and Weapons as they are affected by dispel magic, and that would simply reveal it's magical, not that it's a creature.

While it is an incredibly powerful ability and will almost guarantee to grant advantage on the first attack, after that it grants little benefit unless it flees and attempts to blend in once more. After that your players will be doing perception tests with sharp pointy sticks.

Side Note: As you mentioned Darkmantles in particular, your question seems to imply a concern that them being almost undetectable might place the party in more peril than intended. Their other abilities could sow more chaos than simply the advantage of getting the drop on the players in the first place.