[ He sighs, then shakes his head slightly. To have to explain it all seems odd, but considering the circumstances, he'll bear with it. ]

It is an oath written in blood. It is not a true return to our realm, but where the abyss is strongest, we exchange our blood. It is not a true seal, but it is a ritual to give our oath more weight so that we may not betray it. If either of betrays the terms, then we shall know, and by the laws of our realms, then it is fair enough to take the other's head.

[ He sighs, like this is tedious to explain, then nods. ]

It is simple, really.