Schwenkgott,
Wow, I had a very similar issue (but different!) when playing my son yesterday. Oh aye, and he's 22 - and knows what he's doing, before anyone makes a comment about a "young son"!
But in response to your question, your Forcemaster is not allowed to attack the Priestess; as the knight is guarding in the zone, the Forcemaster MUST attack the knight, and as soon as the Forcemaster damages the knight, the Charm spell drops off.
My issue with my son was different. He charmed my Dire Wolf (remember, who's bloodthirsty) and the Forcemaster was wounded - so what does my Dire Wolf do? To make it worse - my son put his Forcemaster on guard!!! We ended up with the Dire Wolf doing nothing - a seriously confused puppy! (bit like us really).
But that was part of an epic 4 hour game yesterday, where I went through EVERY creature in my spellbook, and lost. And in case any of you reckon we're just really slow, son and I had a game last Wednesday which lasted all of 28 minutes, start to finish. I won that one!!!
But this game just gets better and better - there's a dozen of us at my local club who've gotten into it since January. And 8 of us are gathering for an "all-dayer" in a fortnight's time in a mate's house. Can hardly wait.
Dave I