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".

As the player travels through the different sectors of the UAC base, they can find terminals containing emails between Kronos and other scientists/UAC personnel. These include Kronos demanding that the teleporter be moved to the reactor room, and demanding that Dr. Jensen be detained and stripped of all their equipment.

Guerard's true intentions are finally made clear in Sector 6, 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.

Mother Demon
Kronos bears some physical similarties to the Mother Demon from Doom 64, but in a more skeletal form. Also the appearance shares some similarities to some of the forms of the Goblin/Abaddon pilot concept that appears in DOOM.

Name
The name "Kronos" is possibly derived from the Greek time god Chronos, although a titan of the same name and father of Zeus also exists in Greek mythology. Given the number of geek culture references contained in Doom RPG, it's also possible that the name is a reference to the Klingon homeworld in Star Trek.