Devourer

Devour Foe: Whenever the devourer scores a confirmed critical hit against a living enemy or reduces a living enemy to 0 or fewer hit points, he can steal some of that creature’s energy. The devourer may use his forbidden lore the next round without any fear of backlash. However, when not using this ability, a devourer increases their chance of backlash from forbidden lore from 15% to 20%.

At 10th level, the devourer may use this ability on any foe, not just living foes.