Skylanders Spyro's Adventure soul gem locations guide

Gem #20: Cadaverous Crypt

Unlocks: Hex’s “Skull Shield” Ability

At the very start of the level, you’ll have to use the cannon on the walls to make progress. Before you rotate the cannon, fire it from its starting position to open a new area.

Defeat the spider enemies and descend the stairs to find a Soul Gem.