Let's Read: The Dungeons and Dragons 5e Monster Manual!

[JackPhoenix]

Not that the language discussion isn't interesting, but it isn't really the topic of this thread. Speaking of which...

Darkmantle
Darkmantle is relatively new monster, first appearing in 3e Monster Manual. It's basically Vampire Squid that evolved to fly and live in the Underdark instead of the depths of sea. It's nothing special, but it's the sort of creature I like: it's not yet another humanoid race, or some weird superinteligent monster, it feels like (super)natural part of the fantasy ecology (of course, it's predator, which brings the problem with lack of prey animals in D&D, but that's related to the fact that deers & co. usualy aren't the kind of creatures that would face adventurers in combat).

Art
Like the rest of the entry, the illustration is pretty bland. It shows the Darkmantle just floating in the air, lighted from somewhere above...it does not looks like it's in its normal enviroment. I think it would be better if it was shown hanging on the ceiling (like the smaller, monochrome picture in the corner of the page), or wrapped around someone's head. The triangular tips of the tentacles looks weird, and the tentacles itself would look better if they were more distinct from the "skirt". And what's up with all the eyes? All said, I prefer the 3e version more, however, the lighter brown color may work better as a camouflage, as it's more similar to how stalactites looks.

Purpose & tactics
Darkmantle is an ambush predator, plain and simple. It hangs on the ceiling of some cave, pretending it's a stalactite until something tasty walks nearby. Thanks to its echolocation, it can find the prey even in the dark caverns it lives in, before it can be even seen. Then it drops down, wraps itself around the target's head and try to suffocate it and eat its face. The tactic doesn't work if it doesn't have advantage, and it can't attack anyone else while it's attached...which means it's deadly against lone target, but pretty useless against groups. I'm a little surprised that it doesn't share the damage taken with the target if it's attacked while wrapped around its head, though...it would make it a little more dangerous to take out. If the ambush doesn't work, it can drop its Darkness Aura, like squid would the ink cloud, and either continue attacking the blinded targets while not being bothered itself thanks to blindsight, or escape to try again later. Otherwise, it's damage and toughness is a little weak for the creature of its CR...it's a gimmick monster that's not much of a threat outside its intended role.

Fluff
Fluff is very short...it explains Darkmantle's combat tactics and mentions it is common both in Underdark and Shadowfell. The comparison to bats feels weird, though: bats are very much active hunters, Darkmantle's tactic and prey is very different. The only vaguely interesting part is that some creatures train Darkmantles as guardians, but otherwise, its very boring.

Hooks
Darkmantle is a random encounter monster, or a sort of a living trap, not really something you would base an adventure around.

Perhaps some alchemist or wizard is interested in the Darkmantle's ability to create Darkness aura and wants few hunted down for experimentation? It's not even much of a challenge to catch few of those things live, unless you're level 1...and maybe not even then.

Verdict
I like the monster, however, the whole entry feels a little redundant. With monsters like Winter Wolf being shoved with other animals, I think Darkmantle would fit right in there.