Magic or sorcery is a power in the world of A Song of Ice and Fire. It is regarded with scepticism in Westeros, as it has not been a notable force in the world since the Doom of Valyria. However, it remains more potent in Essos, where practioners known as warlocks and shadowbinders still hold great power.
Magic is said to have declined in the west after the Doom fell across Valyria and the Lands of the Long Summer, but remains extant in the lands to the east. Mirri Maz Duur, a maegi of the Lhazareen, uses sorcery to prolong the life of Khal Drogo, but only as a drooling imbecile. Daenerys Targaryen sacrifices both Duur and Drogo's body on a funeral pyre which results in the hatching of her three dragons.
Melisandre, a shadowbinder from Asshai, uses sorcery to create shadows to kill Renly Baratheon and Courtney Penrose. In Qarth Daenerys Targaryen comes into conflict with the Undying, a guild of warlocks, but defeats them with her dragons. The Guild of Alchemists in King's Landing also notes that their ability to create wildfire has dramatically increased in the last few months (since the birth of Daenerys's dragons, suggesting a causal relationship between the two).