Duriel (a Lesser Evil) can be encountered in Diablo II: Lord of Destruction. He is standing guard in Tal Rasha's tomb, stalling adventurers from chasing the newly freed Baal. His appearance is that of a very large demon with a lower body resembling a maggot while his upper body is more humanoid. His hands are actually large blades and has a giant horn coming out of his forehead.
Prior to patch 1.09 this guy was one of the hardest monsters (if not THE hardest) in the game, greatly feared by hardcore players. As of patch 1.11 however he was nerfed a bit is now more manageable.
As punishment for taking part in the Dark Exile, Duriel was forced to guard Baal's escape path out of Tal Rasha's tomb, where he eventually met his death.
In terms if gameplay, he is very fast despite being large, has an aura that freezes players at intervals, slowing players, and has great strength. Another factor that makes Duriel very hard is the place where you fight him. As soon as you entry his room, he instantly attacks you. There is no exit (except for Town Portals), and the room is very small making it hard to outrun him. Many players relied on minions or mercenaries to buy them time.
There is also an "Uber" version of him found during the Pandemonium Event, that has double life regen, more damage, and stun rating of 1.2 secs. He is known as Uber Duriel. Here is a video of someone killing him:
It is currently unknown if Duriel will be in Diablo 3, but the chances are low since he was killed in Diablo 2.
Some interesting trivia about Duriel:
- Has an item named after him, Duriel's Shell.
- Bears a resemblance to the new Diablo 3 monster Mistress of Pain.
- Looks a lot like the Zerg from StarCraft.