He had learned it from the Black family library, but back then had been too young and unwilling to use this type of magic. Rituals involving living sacrifices affected the soul and mind. They were very similar to the use of unforgivable curses, and with enough practice, they would twist you permanently. Thankfully, his mind and soul were incredibly sturdy and could bear some damage that would simply repair with time. Rituals directly damaging or breaking down the trinity of mind-body-soul were also something he wanted to avoid, as he had seen the consequences of this type of action in Voldemort, who had lost more and more of his sanity with each consecutive soul shard he had made, despite being a veritable genius in magic. But while he wouldn't break or mutilate the sacred trinity, he could enhance certain aspects or simply add to it without suffering terrible backlash.