This is a little odd I must say.
Hmm.......I don't really know. Probably whoever strikes first. Mordo is dark magic, and can basically end anyone who can't counter his spells.
Nate is high end TP and can end anyone who can't handle a mind ****.
Mordo is a magical telepath......so he might be able to hold Nate off enough to offer his soul up to Satannish.