So Athinar's Keeper soul has spared him since the Master really wants a being liek that to serve him, with all of its potential powers tapped...

Of course, it's up to you to decide if the Master managed to resurrect Athinar without making him remember his past. If he doesn't succeed and Athinar is obviously trying to resist, he will most likely attempt some sort of mind control. Best to fake loyalty/stupidity, because fighting the Master in open combat is suicide for just about anyone at this point. You'd have to use treachery and surprise to win.