Throughout the Prologue, defeat Urizen to see a secret ending and get the "Well I'll Be Damned" achievement. This instantly occurs while using the Heaven Or Hell difficulty, as all enemies pass away in one hit, including this boss. You can also do it on the Human difficulty, however it is much harder. To do it the hard way, you need to shoot the red crystal of in charge and avert his attacks. When the red crystal is gone, you can assault him for a couple of seconds on his throne, however he will fix the red crystal within a couple of seconds. You then need to duplicate the procedure up until he is dead.