Sergeant Thomas Kelly is one of the first characters the player meets in Doom 3. He is the ranking non-commissioned officer on the Mars Base, and orders the player to Mars City Underground to find a missing scientist.
His role in the game is significant: as he commands the Doomguy with orders different than those from Councilor Swann, he creates a conflict as to what the Doomguy should do. Eventually, however, Swann's orders are shown to be the best ones as they avoid a possible hell invasion on Earth.
After the demon invasion, Sergeant Kelly makes only one more personal appearance, as Sabaoth, a mutated demon with tank treads for legs, armed with a mechanical claw and Jack Campbell's BFG 9000. He is the third boss out of four in Doom 3 and presents a formidable challenge. As if that weren't enough, a while through as you're battling him, floor plates drop out to reveal pools of lava that instantly kill you if you fall in.
A few tips are to shoot the BFG blasts to disrupt them and fire away with heavy weapons. You can use the pillars for cover, but they will sometimes electrocute you. Keep in mind to not get near Sabaoth, as he will also try to run you over or slash you with the mechanical claw. There are various items around the pillars, such as ammo and medikits of both sizes. When the floor plates start to drop out, watch your step as you keep battling Sabaoth. Once the battle is over, you can take the BFG.