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Small monstrosity, unaligned
False Appearance. While the darkmantle remains motionless, it is indistinguishable from a cave formation such as a stalactite or stalagmite.
Crush. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 6 (1d6 + 3) bludgeoning damage, and the darkmantle attaches to the target. If the target is Medium or smaller and the darkmantle has advantage on the attack roll, it attaches by engulfing the target's head, and the target is also blinded and unable to breathe while the darkmantle is attached in this way.
Darkness Aura (1/Day). A 15-foot radius of magical darkness extends out from the darkmantle, moves with it, and spreads around corners. The darkness lasts as long as the darkmantle maintains concentration, up to 10 minutes (as if concentrating on a spell). Darkvision can't penetrate this darkness, and no natural light can illuminate it. If any of the darkness overlaps with an area of light created by a spell of 2nd level or lower, the spell creating the light is dispelled.
A darkmantle clings to cavern ceilings, remaining perfectly still as it waits for creatures to pass beneath it. From a distance, it can pass itself off as a stalactite or a lump of stone. Then it drops from the ceiling and unfurls, surrounding itself with magical darkness as it engulfs and crushes its prey.