The Chosen is the name given the group of High Priests of Orcus. While most the original Chosen werer destroyed by Kiaransalee (and the backlash of Orcus' deafet at her hands) along with most of the rest of Orcus' priesthood, a small number of Orcus' devotees survived. These three beings make up the whole of the Chosen, and are the primary antagonists the party encounters.
These three are:
Madreus
This guy
Malazar
This Yugoloth
Quag-Namoth
This half-ogre