noun, noun or participle which takes 'suru', intransitive verb
blood oath; blood pledge
A solemn oath or covenant sealed by blood, often in historical, ritual, or dramatic contexts. Can be used as a noun or with する to mean 'to swear a blood oath'.
彼らは血盟を結んで、永遠の忠誠を誓った。
They formed a blood oath and swore eternal loyalty.
その秘密
In that secret society, they are said to exchange a blood oath upon initiation.