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:
- Does an ordinary, completely harmless stalactite gain Unseen Attacker advantage when it attempts to throttle a character within its reach?
- Is it only visually indistinguishable, or should this also fool other senses?
- Should the PCs even get a chance to use Perception (either passive or as a check)?
- Once it's moved, should the stalactite need an action to become motionless again?
- 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...