For the demons themselves, see Great Evils.
The Seven Lords of Hell is a tome found in Act II of Diablo III.
It is found in one of Cain's Lost Satchels. Reading it adds to the Judge of Character achievement.
It reads as follows:
It is with some reluctance that I write on the seven Lords of Hell, for they are the greatest of the demons, and even whispering their names seems to poison the air around me. They are divided into the Lesser Evils (Belial, Azmodan, Duriel, and Andariel) and the Prime Evils (Mephisto, Baal, and Diablo). — Deckard Cain