Catacombs: Horde of Vermin by Sands of Time

The Catacombs : Horde of Vermin expansion is a small box that adds a lot of new twists to an already exciting game. It adds a new monster type, Vermin, represented by brown disks as well as a new Poison mechanic to the base game of Catacombs. Now your heros will face swarms of Sewer Rats, Vampire Bats, Pit Vipers, and other tiny monsters, as well as the ferocious new Wyvern and Remorhaz. It also contains new rooms, some revised, extra scary versions of some existing monsters, and a few new spells and items. Most of the new pieces are the smallest size wooden disks used in the game, packing a lot more Catacombs fun into a small package.

This is an excellent expansion for Catacombs to add variety to your gameplay. There are quite a few new options for monsters and game mechanics in this expansion. I would say that Cavern of Soloth is probably better overall, as it adds additional Characters, but this would be a close second with its array of interesting monster choices.