Religion: does anyone know what is a blood oath in any religion?

A blood oath is similar to a blood covenant. It's where the shedding of blood "seals" the oath or the covenant. I found an article on this subject, part of which says:

When God called Abraham out of his hometown and away from all things familiar, He gave Abraham some promises. A covenant is a kind of promise, a contract, a binding agreement between two parties. The fifteenth chapter of Genesis reiterates the covenant God had made with Abraham at his calling. Except this time, God graciously reassures His promise with a visual of His presence. He asks Abraham to find and kill a heifer, a ram, a goat, a dove, and a pigeon. Then, Abraham was to cut them in half (except the birds) and lay the pieces in two rows, leaving a path through the center (Genesis 15:9-10).

In ancient Near Eastern royal land grant treaties, this type of ritual was done to “seal” the promises made. Through this blood covenant, God was confirming primarily three promises He had made to Abraham: the promise of heirs, of land, and of blessings (Genesis 12:2-3). A blood covenant communicated a self-maledictory oath. The parties involved would walk the path between the slaughtered animals so to say, “May this be done to me if I do not keep my oath.” Jeremiah 34:18-19 also speaks about this type of oath-making.

The article goes on to explain about the blood covenants God made with Abraham. Hope that helps.

A blood oath is where an animal is killed, the oath partners walk through the blood to show the only way they can break that oath is if they are also dead, like the animal. This was established by the God of the Bible.

Bible - > But I say to you, Do not take an oath at all, either by heaven, for it is the throne of God, or by the earth,

but there is a "blood" covenant. it is a kind of promise, a contract, a binding agreement between two parties.

Both GC and JohnH are right. GC gives the correct answer regarding the OT where JohnH gives the correct answer for the NT.