The Bestiary/Catacomb Creatures

Enter "An Abandoned Shack" at 5,4. Go through the trapdoor. To find the majority of the Giant Spiders and Spidra Arcana, try going as north east as you can. You'll find a chasm, enter it. (UPDATE: there is a portal that takes you there directly at 11,13 - Backward Lane)

Catacombs Map