Entangling Vines/Plants and False Appearance

Question

There is some good insight on false appearance at "How should I implement False Appearance?" and "False Appearance, Movement, Surprise, and the Unseen Attacker bonus", however I still find myself unsure about the following scenario.

In ToA, there is a creature called an Assassin Vine that has False Appearance, as well as an action Entangling Vines. The tricky part here is that this action is NOT an attack. The rules clearly state that making an attack gives away your location, but in this case, would taking the Entangling Vines give anything away? What's more, is that unlike the Vine Blight's Entangling Plants (which is also still not an action), the Assassin Vine's Entangling Vines is not centered on the creature acting...

My interpretation of this is as follows:

The Assassin Vines / Vine Blights automatically get a surprise round from being indistinguishable from other plants nearby. On their turn during the surprise round they can take Entangling Plants/Vines for their action. This action, not being an attack, does not leave them "distinguishable" from their environment, so they are still "hidden". Anyone who is restrained by the vines is suddenly in a very bad position against anything other than the Assassin Vines, as attacks against them now have advantage. The Assassin vine gains no special advantage here, as they were already "hidden", and thus had a single attack with advantage before being revealed, and without any multi-attack capabilities, they wouldn't necessarily have anything else to gain advantage on during their second turn... except if the players don't break free by turn three and then, even being not indistinguishable, they still have advantage...

I'm sorry if that was long and rambly, but at any point in there is there something in my interpretation that is not aligned with RAW/RAI?

Answer 1

False Appearance

False Appearance. While the [creature] remains motionless, it is indistinguishable from a normal [thing].

Hiding

False Appearance and Hidden are not equivalent - a creature benefiting from False Appearance is not (necessarily) Hiding. In fact, it is more common for them to be sitting in plain sight.

You can, of course, decide that the creature wants to Hide when it becomes aware of the party in which case:

• It is (momentarily) moving in order to Hide and cannot benefit from False Appearance until it becomes motionless again - if the party is within view this would be a bad thing to do.
• It makes a Dexterity (Stealth) check which is then compared with the passive Wisdom (Perception) of each party member. Those with a high enough passive Wisdom (Perception) know that it is there but, because of False Appearance do not see it as a threat. The rest don't know where it is.

Surprise

See How often during combat can you be Surprised?

Weather hidden or not, the party is "unaware of a threat" and will be surprised. This lasts until each character's first turn is over - there is no "surprise round" in D&D 5e. Note that this may be before or after the Assassin Vine's turn.

Motionless

There is nothing to suggest that Entangling Vines requires the Assassin Vine to move. If it doesn't move then it continues to have False Appearance and if already Hidden, it stays Hidden.

Answer 2

There's no way to be sure whether the Assassin Vine remains hidden in this scenario. As the DM, it's up to you.

False Appearance. While the assassin vine remains motionless, it is indistinguishable from a normal plant.

Simple enough, right? If it moves at all, it's not hidden anymore. So let's look at its Entangling Vines action:

Entangling Vines. The assassin vine can animate normal vines and roots on the ground in a 15-foot square within 30 feet of it.

This is an action, which is to say that it's something the Assassin Vine does. But nothing about it tells us whether the Assassin Vine moves to do it. It could be a purely mental action, it could be something like a spell where the Assassin Vine has to do some arcane gestures - it might even be using its roots to manipulate the other plants under the ground!

So in the end it's up to you as the DM to decide whether the Assassin Vine has to move to use its Entangling Vines action, and therefore whether it can use Entangling Vines without revealing itself.