Mortanius

Mortanius is a character in Blood Omen initially as an enigmatic benefactor and guide who deliniates the player's goals in the game. Later in the game he is revealed to be one of the insane sorcerers whom the main character must kill, and is fought as a boss. Mortanius' motivations are murky and his character is revisited in Legacy of Kain: Defiance. In Blood Omen he is voiced by Tony Jay, while in Defiance he is voiced by Alastair Duncan.

Blood Omen
Mortanius appears in the opening scenes of Blood Omen, first cursing Malek the Paladin for failing the Circle of Nine, and then resurrecting the newly murdered protagonist, Kain, as a vampire. During the game Mortanius is heard more often than seen, speaking to Kain directly through occasional voice-overs. In the end Mortanius is revealed to be the agent responsible for many of the game's pivotal events - the murder of Ariel and the resulting madness of the Circle, and the assassination Kain as well as his rebirth as a vampire. The final battle in Blood Omen takes place against Mortanius, whom Kain kills, and the concluding one is fought against the Dark Entity, a demon-like monster created out of Mortanius' corpse.

Legacy of Kain: Definace
Legacy of Kain: Defiance interprets Mortanius' actions in Blood Omen as the result of a conflict between his own will and that of the Hylden "Dark Entity" possessing him. He is forced to lead the cult of Hash'ak'gik, an organization which his Hylden possessors use to control their human followers, against his wishes. He revived Kain as a vampire with the hope that as the Scion of Balance, Kain would restore the Pillars of Nosgoth and renew the seal on the Hylden. Welcoming death, Mortanius goes to the Pillars of Nosgoth to confront the younger Kain, coinciding with his death in Blood Omen.