…the wide-set eyes, the two breathing mouths fluttering in the shoulders, and the vertically-slitted eating mouth on the central abdomen. […] The vital organs were externally positioned around the central spine, black and pale rubbery sacs which heaved and glistened wetly… and the aliens stank. […] A valve in the central alien’s lower gut popped loudly, spattering the other two with gray-and-white excrement.
Not a bad start for a horrifying Far Realms humanoid.
To start with, misplaced mouths are scary. This alien has three extra mouths, including a big “eating mouth” in his stomach.
There’s a lot going on here, so I’d make this creature a solo or elite monster. I’d make a fight against this monster a fight to stay away from the stomach-mouth. I’d give him long, multijointed arms, and have his main attack be to curl his arms, lover-like, around an opponent (a Grab), followed by an attack on the next turn which presses the victim tenderly against his chest, so that the eating mouth can start chewing.
The external organs are also fairly icky and should be spotlighted. I’d make them be separate targets which can be attacked by the PCs. A hit causes an explosion of vile fluids (a burst attack on everyone nearby) and ongoing damage to the monster as it leaks ichor.
The excrement is maybe too much, depending on how much you want to gross out your players. I’d change it to some vile-smelling acid, but keep some of the description the same (“A valve in the creature’s gut pops loudly, spattering acid on… [rolls dice]”
All in all, not a creature the PCs will want to fight a second time, especially the fighter who was half-chewed up by the monster’s torso teeth.