The last in our list is Belial, one of the two remaining Lesser Evils along with Azmodan. Belial was also one of the two Lesser Evils to instill the war that caused the Dark Exile, with the other being (no surprise) Azmodan.
Many things about Belial are unknown as of now, but we do have some concept art and Lore references.
What we take from lore is that he has a servant called Xazax, loyal only to him.
In terms of appearance, we can only base on the following concept art so far:
From the concept art and a beta portrait we can see that Belial looks very skeletal,has two mouths on each side of his face, and four eyes.
This guy has been confirmed to be in Diablo 3's second Act, however it is unknown what powers he will have.
More information will be revealed as Diablo 3 comes out.