Blood magic

Blood magic is a form of sorcery, forbidden among the Dothraki. Mirri Maz Duur was a known practitioner.

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. She takes Drogo's horse into his tent and slits its throat as the ceremony begins. Qotho becomes enraged as Daenerys attempts to save her husband and tries to attack Daenerys, but Ser Jorah kills him. Daenerys starts to go into labor, but none of the Dothraki midwives will help her, thinking she is cursed. With no choice, Jorah takes Daenerys into the tent where the blood ceremony is taking place.

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, a deformed monster with bat wings. The khalasar has moved on, leaving a small group behind. However, Drogo lives. Mirri Maz Duur takes Daenerys to see her him, only to find him lying in a catatonic state. Daenerys demands to know why, and Duur explains that when Daenerys saved her, she'd already been raped three times. She explains that she has prevented the Stallion Who Mounts the World from causing more destruction and suffering