In the Tosa/York multiverse to be Sword Banded is when warriors enter into a agreement, usually for life, to always fight beside each other. To never betray each other. To share glories and failures.
The tradition developed among the Centaur & Human tribes of Paradox but spread to many different cultures. It is a source of pride but also jealousy.
The pinnacle of this tradition are Duke Jake Whitetail and duke Kirgor the Red.
Another pair of legends are Sir Cantor and Sir Janak.