Does the fight involve cipher spells like desintegrate? I don't believe vithraks do this, but maybe Sissak himself does those things.
Anyway, i have tested the charm effect to death thing more thoroughly now by using summoned wurms and eder as guinea pigs, it happens in 90% of the case when the effect expires the affected character simply dies, sofar only spores in sanctum does this, normal dank spores is not a problem.