Mm, Nexus is the world that the assorted players at tf_ic_prompts play in. It is a multiversal location, entry and exit controlled by the Powers That Be^TM, also known as the Royalty Beyond (aka the players). Characters from all over the TF multiverse appear there, survival is a fair game and the odd luxury (like a karaoke bar) will crop up from time to time.

Since all the characters trapped in Nexus do not remember anyone mentioning the place, general consensus is that no one remembers Nexus and events that take place in it after they leave.