Related to How should I implement False Appearance? but that does not answer my questions about surprise and movement…
A Darkmantle has the "False Appearance" property which allows it to be indistinguishable from an ordinary stalactite while it remains motionless.
Suppose there is a Darkmantle on the ceiling of a cavern, 30 feet high. A party of adventurers walks in and cannot perceive the Darkmantle. It decides to attack. Since none of the party members can notice the threat, I believe they are all surprised at the start of the encounter, and unable to act during the first round.
PHB p.189:
Any character or monster that doesn't notice a threat is surprised at the start of the encounter. If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.
But the party members are not in reach. On its turn, the Darkmantle needs to fly down and then make its attack action. Once it begins flying, it the party will most likely notice it. How the movement interacts with Surprise and Unseen Attacker is the part I'm most uncertain about.
- Does the movement at the beginning of its turn ruin the surprise? (I think not)
- Does the movement at the beginning of its turn ruin its opportunity to get advantage on the attack roll as an Unseen Attacker? (I think maybe, depending on Stealth: see #3)
- Can the Darkmantle attempt to fly stealthily (Stealth check contested by each player's passive Perception) in order to maintain its Unseen Attacker advantage? (I think so)
How does this all play out for the DM? Suppose one member of the party's passive perception beats the Darkmantle's stealth as it flies down. Then do I (as DM) say, "Suddenly you, Aldore, notice one of the stalactites swooping down to attack. Everyone roll initiative, but because you didn't notice it before the start of its turn, none of you can take actions this round." And if its attacking anyone other than Aldore, does it get Unseen Attacker advantage on the attack roll, or is Aldore's shouted "look out!" negate that advantage? (Or can Aldore technically not shout a warning since he can't act? Often we do allow PCs to talk out of turn, but in a case of surprise, we'd go by RAW.)
Best Answer
The combat begins when the Darkmantle decides to attack. If the players don't notice the Darkmantle at the beginning of combat, they are surprised:
So, thanks to False Appearance, the Darkmantle has a Surprise attack. Also, when the combat starts, the adventurers don't know that they're in a combat situation and so are not scanning the entire space all the time, unless previously stated (they are not combat-aware), so the Darkmantle remains hidden until its turn. However, all characters that are higher in the initiative order than the Darkmantle are no longer surprised at the end of their turn (or, better say, the "Surprise" property is no longer applied to them)! They are thus able to take reactions. For reference, this answer to another question (also linked in the comment of this question) breaks it down in full detail.
The Unseen Attacker advantage, however, depends on the DM's ruling, as the RAW are not specific about this.
As far as Darkmantles go, they could approach their target unnoticed. They don't have to fly, they can just drop down. One could reasonably fall unnoticed as easily as dropping a curtain on top of someone. Even if someone would notice the creature as it was falling, they'd only get enough time to look up and see the horrid array of teeth approaching swiftly.
If you still believe your Darkmantles are unable to automatically Batman your PC's, then a Stealth check is appropriate, and should be made during the Darkmantle's Move action. In either case, the Surprise Attack persists, it is only the Unseen Attacker advantage that is at stake.
Keep in mind that the target of the attack must be taken into account. If the Darkmantle attacks a character that was higher in the initiative order, the character can take reactions (like Tempest Cleric's Wrath of the Storm), regardless of whether the Darkmantle remained hidden during its movement or not. The very act of attacking reveals it:
However, if the trigger for their reaction is before the attack, the Darkmantle's visibility has to be taken into account.
If the Darkmantle attacks a character that's lower in the initiative order, the character won't be able to react until the end of its own turn. So, even if his reactions trigger "on hit", he won't be able to use them. However, if the characters that went before the Darkmantle have a reaction that triggers "when an ally is attacked", they can take that reaction (again, because the attack reveals it).
There is only the final, special case to consider, where the Stealth check is only partially successful and one or more characters notice the Darkmantle as it moves in to strike. It is fully within their ability to warn the others of the threat, as they can talk out of turn (within reason). However, as it's the Darkmantle's movement that triggers the Stealth check, the players that were surprised at the moment of the attack still remain surprised. Whether the darkmantle retains its Unseen Attacker advantage or not is up to the DM and his opinion on how fast a character can react to a warning.
From a role-playing perspective, if the Darkmantle remains unseen the adventurers either didn't notice the attack until the moment it hit, or they noticed it too late to do anything about it (including removing the advantage).
Here's an example of how that could be acted out:
Now, if the creature missed the attack (either by lacking or despite the advantage) you could have it be because the character noticed it and dodged in the last second, or you could make the Darkmantle comically flop on the ground in the middle of the party. Your choice :)