This room's floor, walls, and ceiling are covered in a mosaic depicting a solar system. None of the planets or constellations match those familiar to the party.
This room is home to a giant trapdoor spider who – if not discovered by the party – will wait until the party is leaving the room before emerging from a cutout in the floor and silently attacking the last member of the party to leave the room, attempting to do so without the rest of the party noticing – at least until the party member has been dragged back into the spider's tunnel and the cutout is safely replaced.
Use Dexterity and Wisdom checks, grappling rules, saving throws, called shots, or whatever your system uses to adjudicate the spider's success. Other than that, stat as a standard giant spider.
The corpses of previous prey are buried in the silken walls of the tunnel. PCs who search them can find assorted coinage totaling 5d100 gp but will need to deal with 2d4 lesser giant frogs that serve as pest-control for the spider.
Giant Net-Casting Spider
Stat as a normal giant spider. Hangs out on the ceilings and drops a sticky web net down on a character. Net does no damage but net attack hits at +4. Anyone caught in the net should have a really hard time getting out of it. The character caught in the net is pulled up to the ceiling so only ranged attacks will do anything – and they have a 1/2 chance to hit the character rather than the spider.