Kal'Kuzak, a Demon-God from hell has been observing Doom guy throughout his onslaught of hell. He's taken an interest in him and admires the way he slaughters his own kind. 

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As a test, Kal'Kuzak invites you to his Tower of Pain. An invention he made for the sole purpose of pain. If you can make through all floors of the Tower he promises you will be rewarded handsomely. Do you accept the challenge?