blood brother

noun3

1. •
   An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures.

   • "He is a blood brother and an oath brother of Odin, highest of the gods." — 2017
2. •
   An unrelated male considered to share such a strong relationship generally.
3. •
   A brother by blood: a brother-german or half-brother, as distinguished from step-brothers.