According to the gamers who contribute to the Diablo Wiki, Grotesques in this game "seem to be doll-like entities made of human skin sewn together."
But wait, there's more:
"...the grotesque are basically living pinatas. They are fat, white, humanoid monsters, who run up to a player who comes into range, take a single hit, bend over backwards, and rupture into a massive fountain of wriggling silver eels."
For your viewing pleasure, here is a clip of this precise event taking place.
"There seem to be far more eels within a grotesque than their volume would permit, but such is the nature of magic. The eels, properly called Lamprey are wriggling creatures that must be killed lest they consume your character's toes, but they seem to die very quickly, and on the whole the Grotesque-Lamprey combo looks more inventive and amusing than dangerous."
If you are getting a sense of déjà vu from this description, you are not alone. The Grotesques in Diablo III are very similar to the Grotesques in Hellgate: London. Both are described as figures whose bodies are stitched together from various pieces of flesh. Both explode when attacked, releasing a dangerous surge of eels or maggots. They also somewhat resemble each other at close quarters, each with multiple heads emerging from bulbous yet sinewy bodies crisscrossed with Frankensteinian scars. Here is the official art for Diablo III's Grotesque. Click for a closer look.
Two separate grotesques... why the resemblance? As usual, the good people of the Internet have the answer:
"Grotesque are extremely similar to a monster type in Hellgate: London also knows [sic] as Grotesque. In Hellgate, Grotesque are shambling, twisted mounds of demonic flesh that, upon death, explode and release numerous zombies and Death Maggots. Given that Hellgate: London was created by former members of Blizzard North, which made Diablo II and started work on Diablo III, one might speculate on some connection between the very similar monster types seen in the two games."
Most importantly, game producer Bill Roper (who was heavily involved in the creation of the Diablo series) left Blizzard Entertainment to begin making Hellgate: London.
I always find it interesting to observe how similar visions of grotesqueness are perpetuated in different texts. In this situation, a solid connection can be made between the two constructs. Individuals who share a notion of grotesqueness that is embodied in misshapen, composite and explosive human flesh have applied their definition while creating multiple texts, resulting in Grotesques that are similar in both appearance and behaviour.
Ah well, back to work...