Elric of Melnibone. Pretty grim '60s-'70s fantasy Michael Moorcock. Wikipedia gives a pretty good summary of the premise.

Elric is the last emperor of the stagnating island civilization of Melniboné. Physically weak, the anemic Elric must use drugs (special herbs) to maintain his health and vitality. From childhood, he read freely in the immense royal library and learned of the world outside the Dreaming Isle. Perhaps due to this in-depth study, unlike other members of his race, Elric has a conscience. He witnesses the decadence of his culture, which once ruled the known world, and worries about the rise of the Young Kingdoms populated by humans (Melnibonéans consider themselves separate from humanity), along with the threat they pose to his empire. Because of Elric’s introspective self-loathing and hatred of Melnibonéan traditions, his subjects find him odd and unfathomable. In addition to his skill with herbs, Elric is an accomplished sorcerer and summoner. As emperor of Melniboné, Elric is able to call for aid upon the traditional patron of the Melniboné emperors, Arioch, a Lord of Chaos and Duke of Hell. Elric’s discovery of the sword serves as both his greatest asset and disadvantage. The sword confers upon Elric strength, health, and fighting prowess, allowing him to do away with his dependence on drugs, but it must be fed by the souls of intelligent beings. Most of Moorcock’s stories about Elric feature this relationship with Stormbringer, and how it—despite Elric’s best intentions—brings doom to everything he holds dear.