Didn't want to post until after the session in case one of my players might have seen (I don't think they're on this forum, but just in case...).

The fighter of the party was tossed in jail the session before last and the last word said to him by the lord of the town (and later by a party member who'd negotiated his release and was relaying the terms) was "rot". He was an evil character and his point of view was he should be able to do ANYTHING he wants because he's helping the town with the Keep. No one felt they could trust him, so he was left in the jail (what was originally going to be a one-night stay got extended since he was so hostile to anyone trying to negotiate his release). Some mysterious entity got in touch with him and offered him a chance at revenge, nifty magic items, and command in an army for unquestioning servitude. He agreed.

So during the next combat, two battles happened at once: the first of them was the regularly scheduled "next room of the Keep" battle. The second battle, happening simultaneously so both battles used the same initiative order, was the fighter cutting his way through the guard. Once he was free of the town the character became mine and the PC became the newest NPC (and a nemesis of the party). The player got to play his old fighter breaking out of the jail in one battle and his new character helping the party in the other. Interestingly, the players were rooting for the fighter to escape because they liked the idea of a nemesis so much. I didn't have time to finish the layout so I had to go part 3D, part 2.5D. I'll post pictures later.