A note, possibly more for myself, the arrows are not fey slaying arrows (magically), they are arrows dipped in fey killing poison. The Killoreen would have missed his Fort save as well as having missed the Will save (poison is Fort, Slaying is Will), so it doesn't matter for that.
Descriptively, he does not turn ashen and his soul does not coldly leave his body. It is a more terrible, blood spurting our of mouth poisoned death.