The Tomb of Tarraketh
This is an incredibly deadly dungeon that will strain the limits of any party that dares to tread within. I consider this to be a challenge dungeon of sorts, and I would suggest it as a dungeon to throw at players if they ever want to run a gauntlet with new characters. This dungeons is filled to the brim with traps, puzzles, and vicious creatures. It is also quite a long dungeon - be prepared to spend at least two sessions working through the pathways of this horrifying Tomb.