Blood magic is a form of unearthly sorcery, forbidden among the Dothraki. Mirri Maz Duur is a known practitioner.
In the series
Baelor
Mirri Maz Duur promises Daenerys that she can save Drogo's life using blood magic, but she needs to sacrifice a life to save his own. She takes Drogo's horse into his tent, violently slitting it's throat as the ceremony begins. Some of Drogo's warriors become enraged at what they see as Daenerys's attempts to interfere with the natural way of things and Qotho tries to attack her, but Ser Jorah kills him. Daenerys starts to go into labor, but none of the Dothraki midwives will help her, thinking her cursed. With no choice, Jorah takes Daenerys into the tent as the ceremony continues.
Fire and Blood
In the wastelands beyond Lhazar, Daenerys wakes up to face terrible news from Ser Jorah Mormont. Her son, Rhaego, is dead. He was stillborn, and a deformed monster besides. The khalasar has also moved on, leaving them behind. However, Drogo lives. Mirri Maz Duur takes Daenerys to see her sun and stars, only to find him a catatonic vegetable. Daenerys demands to know why, and Duur explains that when Daenerys saved her, she'd already been raped three times. Now her 'stallion who mounts the world' can burn no cities and slaughter no innocents, all with the aid of the blood magic.