Guild Hideout is a two-level dungeon zone in Act V of Diablo III, located beneath and accessed from Westmarch Heights (only in Adventure Mode).
Its design is based on the unused Westmarch sewers zone, The Cesspools.
Inside, as its name implies, one may find the Thieves Guild assassins, thieves and summoners, including three unique leaders of the Guild. It also has a Resplendent Chest on each of its two levels.
Clearing this dungeon, or killing said unique monsters, may be nominated a bounty.