Different classes draw on different power sources for their abilities. The power sources used by the "Player's Handbook" classes are arcane, divine, and martial. Arcane classes gain magical energy from the cosmos, divine classes receive their power from the gods, and martial classes draw power from training and willpower. The "Player's Handbook 2" introduces the primal power source, which draws power from the spirits of the natural world and features transformation as a theme. "Dragon" No. 379 included the Assassin class, introducing the shadow power source. The "Player's Handbook 3" introduced the psionic power source, which draws power from the mind. "Player's Option: Heroes of the Elemental Chaos" introduced builds which use the elemental power source.