Kronos

In Doom RPG, Kronos is the second-to-last boss that the player must defeat. He is the mastermind behind the invasion of the base, somewhat analagous to Dr. Betruger in Doom 3.

Kronos was a UAC scientist working on teleportation research. He supposedly travelled to a dimension of pure evil where he combined human technology with demon flesh to create the Cyberdemon. In Reactor, the last level of Doom RPG, it is revealed that Kronos has been changed into a Demon.

After the UAC discovered Kronos' research, he was fired and his research buried, but he returned in the guise of Dr. Guerard. Dr. Guerard is encountered early in the game in Sector 1, where he helps the player. However, he sometimes behaves in a suspicious manner, opening doors in apparently impossible ways, for example. Some of the scientists express their suspicions about Guerard, some believing that he is "working for Kronos".

Guerard's true intentions are finally made clear in Sector 7, when he leads the player and Dr. Jensen into a trap. Guerard is encountered again at the entrance to the reactor, where he transforms into Kronos. The player must defeat him to reach the portal and shut it down.