I (Mike) had pre-chosen the open groups for the Overlord to control: Shadow Dragons, Fire Imps, and Hybrid Sentinels. The Imps and Sentinels were from an expansion box, so the Overlord had a bit more power than usual.
But over time the heroes were beaten down. The dragon eventually died but a new Fire Imp stepped in to grab the key and run for the door. The Heroes were down to the final few turns and attempted to close the final two portals. Jain failed several attempts at one but it didn't matter as the heroes never got a shot at the final. Our drunken dwarf Grisban the Thirsty (who's Whirlwind landed several attacks this week!) finished his last turn in reach of portal control but had no time left to attempt the closure (I can't say enough good things about the balance in this game for most quests).
The Overlord ran out the door and claimed victory.
In related news, this quest was a big one and it ran late. The owner of the store we play at offered to let us keep playing (we did manage to finish before normal closing time) and considers us regulars!