When two people are born at the exact same time, they become soul bound. Soul bounds can feel each other's pain, get the same wounds and die when their soul bound dies

Shigaraki knew soul bounds existed but didn't think he had one. That was until he felt the first soul bound pain. He was sitting on his bed when he was 9, when he felt a sudden sharp pain on his arm. Looking down he saw that he had a long bleeding scratch. Since then the wounds come about more frequently, bites, scratches, cuts and bruises, all along his arms, back, stomach and sometimes even his legs. He never got scars, since it was from his soul bound and not him but he was still very worried about his soul bound

Now he is 20 and leading the League of Villains.

When looking at new members for the League, one of the people there caught his attention. They had scars all over their arms, exactly like how his soul bound would be scarred based off how many bound wounds he got throughout his child and early teen days