So I've got to thinking about bringing back some of my villains that got away. Here's my list:
Kazmojen - last seen fleeing into the underdark. If the Blue Duke gets killed in the Battle of Redgorge, he's our new captain of the city guard.
Skaven Umbermead - Got away from the Kopru ruins, but the party killed the mooks tasked with hauling his books up. First contracted the Last Laugh to rob the party wizard's room and get the books back. They failed. He led the second strike, killing the wizard's mimic and reclaiming his books. Probably going to come back again as a Cagewright wannabe in Lords of Oblivion.
Gutterut, the Quasit made something of a nuisance of himself at the end of Flood Season. After trying to get the spider climbing fighter to fall (and getting bashed for his trouble), he decided to use his wand of water control to raise the water in the lake outside the Kopru ruins. Made for a nail-biter to see whether the spider-climbing fighter could get everyone to safety before the water got high enough to let the kopru pull the plate-mailed cleric under. They made it just in time. Gutterut will probably be back with Nabthateron's army to cause some mischief on the walls of Redgorge.
Dhorlot, the black dragon never even met the party. There are plenty of other dragons, so he won't be coming after them.
Olomasta, half-fey green hag fled after her two sisters were killed. She complained to Nabthateron about the party, but when she told Nabby that the party now had Alakast, well, he ripped her in half. She probably won't be coming back (mostly because I think hags are a pain to run.)
Nabthateron will definitely be back, and leading a demon force to finally obliterate Redgorge.
So who got away from your party and how do you plan on bringing them back?