Don't know how unique this is, but it's what I got:
A symbiotic mob; a beautiful women contained by a slime. They put up a facade, of a women who was eaten by the slime, screaming for release. Using her looks and cries, she attracts prey for the two to feast on, while the slime slowly encapsulates the opponent. They would spawn on Sea Foam. Gelatinous forms that generate in clusters in Ocean Biomes. They could drop some sort of dark magic necklace that allows the player to be consumed by a slime, assuming the abilities of the mob? [I have no clue]
Their attacks would comprise of so:
The girl changing the color of the slime, applying a different potion effect on contact.
The slime spitting miniature slimes that swarm the opponent
The slime leaping onto a foe, suffocating it.
The slime expanding to consume enemies within a melee range.
The girl projecting a cube of slime that encapsulates the opponent.
Every once in a while the girl will force her head out of the slime to taunt the player, this is when she can be harmed.
The slime is invincible, but the girl only has 60 health.