Skylanders Spyro's Adventure soul gem locations guide

Gem #21: Cadaverous Crypt

Unlocks: Trigger Happy’s “Ignite Hellfire” Ability

You’ll be able to spot this Soul Gem from where you grabbed the previous one, but it will be a while before you can reach it.

Continue through the level until you reach The Catacombs. You will know you’re close when you reach this battle gate.

Cut down and to the left to find the Soul Gem you saw earlier.