I consider this a hidden treasure, but maybe everyone already knows about it. This is a picture of Tweek, sleeping in his den which is under the Tavern In The Mists in The Veiled Stair. If you go around the west side of the building towards the back you can walk right underneath the tavern and check out his lair. Or just go under the front porch by entering next to the stairs. Which is probably easier, but I just now discovered it was an entry point. Either way, meet Tweek.