To find the dragon's various powers, follow these hints:

Electricity Power: In front of Bonegrinders lair after Mesadril is dead.
Ice Power: Floating above Maultok's arena after you get Arokh.
Darkness Power: This is obtained after you defeat the two evil dragons that the traitor awakens.
Spirit Power: Appears after you defeat the Bone Dragons and the Necroman. It's outside his tower.