Sarevok and his three acolytes (Tazok, Angelo, Semaj) are joined by a fourth: Diarmid, who the party briefly encounter at the very end of chapter 6. He's a fighter/mage rather like Angelo.
More crucially, Sarevok is the beneficiary of a spell which lets him draw protective strength from his followers. You won't even be able to hurt him until at least one is dead, and killing him requires all four to have died. (Not terribly original, I admit, but in the original battle you might as well just ignore everyone else and go straight for Sarevok. This way it's a proper fight against a team of opponents. And hopefully the added dialogue is smooth enough not to make it feel contrived.)