The Warlocks are a group of magic practitioners based in the city of Qarth. They gather at the House of the Undying and are recognizable by their blue lips, caused by their custom of drinking a substance known as shade of the evening.
After centuries of decline, the magical power of the warlocks grew stronger in the weeks following the hatching of Daenerys Targaryen's dragons. Some time after her arrival in Qarth, the Warlocks, through Pyat Pree, form an alliance with Xaro Xhoan Daxos to kidnap the dragons and Daenerys herself in exchange for their aid in making Xaro the King of Qarth.
Since their magic was strongest in the presence of the dragons, and they were strongest in hers, they planned to keep both her and the dragons prisoner in the House of the Undying. Daenerys, however foils their plans when her dragons kill Pyat Pree. She escapes the House of the Undying afterwards and, shortly after, leaves Qarth by ship.
Several weeks later, a warlock with the appearance of a young girl attempts to assassinate Daenerys when she arrives in Astapor. The attack, however, is foiled by Ser Barristan Selmy, though the would-be assassin escapes.
- Warlocks at A Wiki of Ice and Fire (spoilers from the books)