The Campfire

You enter this area after defeating the Nuclear Throne boss.

It resembles the Character Selection Screen, but all the playable mutants save for your character are dead and you see their corpses (besides Y.V.'s and Horror's as they don't leave a corpse behind) and the campfire is no longer lit.

Four I.D.P.D. portals will open as your character spawns in. After killing all the I.D.P.D. troops, the area changes into a floating piece of land and you fight the Nuclear Throne's Final Form.

By defeating the true last boss you proceed to loop the game, teleporting back to the Desert but this time with increased difficulty.

Trivia

 * Playing as Fish, you will find his guitar here, which can serve as a high damage melee weapon.
 * The boss theme for Throne II is "This Is It"