as it goes in the books the doomguy ( if anyone wonders he is Corporal flynn taggart but is called fly, united states marine corps; serial number 888-23-9912) looks through a blood smared crack (in which his buddy arlene sanders went through) and sees outside (dont ask how hes seing outside) sees crossbones and an arrow he cant get through the crack so he unslings the rocket launcher goes down an elevator and sees 2 hell princes he ran to the other side of the room and fired 6 rockets at one it died then he ran up to the other and dropped a rocket down it's throat and ran when the rocket blew it knocked him out and when he woke up the walls of the chamber were crumbled and he was outside and saw a warpgate (in the game u dont see it) he steps through and he was on fire but he wasnt hurt by it. the fire burned away his clothes and his weapons. he went around and found a gun laying under the body of a person from fox company (the guns called a sig-cow i dont know if its in the game) and thats what happens between the levels.... to sum it up the black room is really just the warpgate. if anyones wondering i read the book cause we had to do an book report on a fictoinal book, i dont think my teacher liked it thought (she was in her 60s) i got a good grade though
