At the end of White Plume Mountain, Schick writes that if the party of adventurers is still strong enough to continue, they might encounter Lord Keraptis—at great peril or his henchmen Box and Cox. He advises that a good Game Master (GM) can just figure out what that encounter might look like.
TRG1 was my initial response, in the form of a dungeon, to Schick’s challenge back in the mid-1980s. For some reason, I ended on a similar note as Schick and left the ultimate ending to the GM as the party never encountered Lord Keraptis nor his minions.
In the Chambers of Keraptis (TRG1-3) nearly forty years later, finally brings that adventure to a close.