Echolocation. The darkmantle can’t use its blindsight while deafened.
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.
While attached to the target, the darkmantle can attack no other creature except the target but has advantage on its attack rolls. The darkmantle’s speed also becomes 0, it can’t benefit from any bonus to its speed, and it moves with the target.
A creature can detach the darkmantle by making a successful DC 13 Strength check as an action. On its turn, the darkmantle can detach itself from the target by using 5 feet of movement.
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.
Some fun little guys! Have definitely been overlooking them, hope I find a way to put them in an encounter soon
these guys don't have darkvision?? can they even tell when they're in a patch of magical darkness if nobody lights a torch? say a suspicious lone goblin 20ft away in natural darkness shoots one arrow at two Darkmantles and Darkmantle-A decides to activate Darkness Aura, then will nearby Darkmantle-B perceive that it is now obscured? it's presumably not blind, just low of intelligence. how does Darkmantle-B react (unwounded, sensing prey, etc) to increase it's chances of attacking with advantage? does it Hide knowing that it is now heavily obscured? does it attempt to Hide or stay hidden without regard for the active observer's darkvision (potentially resulting in only light obfuscation (and a failed Hide)) or the aura, trusting that any darkness is good darkness? does it pop it's own daily Darkness Aura to ensure cover? does it attack confidently right now, again ignoring the prey's darkvision (and thereby lacking hidden status and thereby lacking advantage)? or does something else happen??
SGUID
I understand this is just over a year old, but wanted to add some input to this.
You may understand this now so ignore all of this completely if so, just a tidbit of information.
AC isn't the ability to hit it, just pierce the armor/defenses of the creature, whether it be dexterous speed or thick hide, so in the future (unless you've changed your style) I would highly recommend that be the argument, that a blinded creature has disadvantage on attack rolls against a creature, even if it is attached to their skull, due to their inability to properly aim or get a good angle to pierce its defense.
It's your table so it's your ruling, but making the choice to hurt themselves or their friends is looked down on very often and can ruin the experience almost always for the players. I hope your future games are filled with fun and classic D&D ridiculousness.
They have Blindsight out to 60ft. This means that regardless of light conditions, they are able to identify their surroundings and pinpoint creature's locations so long as the creature isn't behind total cover (completely behind a wall or door for instance) so long as those surroundings/creatures are within 60 ft. This is usually accomplished by relying on a different sense such as hearing via echolocation which is what I believe this creatures uses.
I appreciate the comments regarding my previous encounter post but even after a year, I still think I would have run it the same way. The rules of the creature suggest that once a darkmantle is attached to their head, the creature is blinded and unable to breathe. The blinded condition says that attacks against the blinded creature have advantage and the creature's attacks have disadvantage. It does not state that attacking the darkmantle on your own head would not also provoke this disadvantage. Interpret this as, you are attacking carefully so as to not deal damage to yourself while slicing through thin webbing, or bashing a small creature with a large mallet while it rests on your skull. You can't just go full bore on a creature wrapped around your cranium, so the disadvantage makes sense. Their comparatively low AC of 11 means that even with relatively low rolls, it shouldn't be all that difficult to hit them even with disadvantage.
So when my players raised the objection, I gave them the choice. Attack full bore at the parts that are wrapped around their face, in which case the creature would take full damage but half would also be applied to themselves, or attack the part resting on top of them which is harder to see, reach, and attack causing the normal disadvantage but none of the damage would be applied to them. I think that's fair and permits greater choice to the players, all of which were veterans at the game as we had been playing a prior campaign for over 3 years at that point.
Still appreciate all the input, but honestly, I think I stand by my previous decisions.
If you go to the Under dark, these make a great obstacle!
B A L L S
"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."
"they fill an ecological niche similar to bats"
Okay, I think someone has to take another look at the ecological niche of bats, because its definitely not suffocating medium sized humanoids.
No kidding. How the heck does magic clouds of darkness and surprise suffocation land you in the same “ecological niche” as the four ounce, moth and mosquito eating bat.
Interestingly enough, on page 280 of the DMG, under Damage Transfer, the Darkmantle is the listed example monster. It would therefore, "Double the monster's effective hit points," and also "add one-third of the monster's hit points to its per-round damage."
Looks like at one time, at least, the Darkmantle DID have the Damage Transfer property.