5th Edition - Maggot Lamia

Maggot Lamia (CR 4, 1100 XP)

Large monstrosity, chaotic evil
Armor Class 13
Hit Points 97
Speed 20 ft.
Str 18, Dex 13, Con 15, Int 14, Wis 15, Cha 16

Skills Deception +7, Insight +4, Stealth +3
Senses darkvision 30 ft, passive Perception 12
Languages Abyssal, Common

Multiattack. The lamia makes two melee attacks.
Sword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (1d8+4 piercing damage).

Intoxicating Touch. Melee Spell Attack: Hit: The target is magically cursed for 1 hour. Until the curse ends, the target has disadvantage on Wisdom saving throws and all ability checks.

This is hands down the best image Google showed me for "lamia."
It's not what I was going for with this monster but hey, it's too good.
 By John Suarez.

Similar to their mythological counterparts, lamias are an amalgam of the harpy, the siren, the succubus: essentially, they're yet another temptress. We're spoiled for choice when it comes to temptresses in D&D. The maggot lamia is a variation on this theme, different in that it promises physical metamorphosis, not sexual fulfillment, to those who serve it.

Maggot lamias are abhorrent. Their upper bodies are grotesque women who would've been great extras on The Hills Have Eyes. They have the lower bodies of maggots. They also lack the ability to change their appearance, and so must always be seen for what they truly are.

Their best method of defense is the secretion of a thaumaturgical pheromone that causes humanoids to feel an intense desire to serve. The maggot lamia instinctively seeks to transform herself into something else, and the beings which fall under her command take on this desire as well.

Successful maggot lamias establish cults which keep them fed—and they prefer the flesh of sentient humanoids. At some point during their life cycle, their flesh begins to harden; the maggot lamia will soon seclude herself to undergo a potent metamorphosis into—well, that's up to you. In classic D&D, chasmes, being fly demons, would work perfectly. If you're playing D&D 3.5, you could slap the Insectile template on her, bump up her hit dice, and give her a fly speed. Really, the possibilities are endless.

From a design standpoint, the maggot lamia (or a clutch of maggot lamias) serves as the 'big bad' of a low level cult. They can unite a wide array of humanoids under their control, adding mystery and variety to a dungeon crawl, and if they're not dealt with in a timely manner by the PCs, they can 'upgrade' and become even more dangerous the next time they're encountered.

Since they're always repulsive, and the beings serving them grow to want to serve them (because they, too, aspire to become more), exposing them as monsters does nothing to disillusion their followers. They are monsters who want to become more monstrous; they are evil, and they want to be worse.