The spell states that you can cast this when a player is about to roll a die, the question is can this be used when a player is rolling the attack roll of a monster? We argue about this since it is not the character's turn but he his still rolling a die. We allowed it and as such a player was killed by the vampire prince.
Funny thing happenned with the Misfortune spell in our last game, I had twice the spell in possession, a roll a 1 for my movement, the player I wanted to kill was 1 square away from the reaper, I rolled the die didn't get a 1, used a fate to reroll and cast Misfortune on me, and then on the other player, very upset player