Caim made his pact with Angelus after being mortally wounded and discovering bound dragon in castle's hall. Despite initial antipathy they decided to forge pact to get their revenge. He gave up his voice for the pact.
Leonard attempted suicide after his brothers were killed by empire but was stopped by fairy, who convinced to forge pact, to avenge his brothers. He gave up his sight for the pact.
Arioch witnessed murder of her child by empire, which led to her going insane and starting to devour children. That led to her capture. She was freed from capture by salamander /undine and gave up her ability to birth.
Seere had his home attacked, and his mother sacrificed herself to protect him, which was made into catalyst to forge pact with golem. It costed him his time, meaning he'll never age.