Cassandra Nova uses her psychic powers to enslave Rachel Summers. Putting a collar and a leash on her, she literally makes her a pet, and uses her as an attack dog against her enemies. Constantly de-humanizing her, and forcing her to fight Jean Grey, she's eventually KO'd by a powerful blast. In order to save her from being Cassandra's slave, Jean Grey has to offer herself up in her place.
Source: Comic