From Diablo Wiki
The Edyrem are a group of common townsfolk who united with Uldyssian, the central protagonist in the Sin War novels. Uldyssian (who also gave them their name) with the help of Lilith (disguised) trained this large group of common villagers from various cities to tap into their Nephalemic heritage and expose latent abilities and powers, some more successful than others and they proved to be quite powerful. A power which Lilith wanted to control and which she also did during a time. In the battles against The Cathedral of Light and the Triune many Edyrem were equal if not stronger than the priests of the churches.
The Edyrem appear in the following books:
For more information, see Nephalem