It was in the Frozen Horror scenario. I think I've also had it in the Chaos Realm.
I don't remember any additional details of what happened the first time, but I think the second time the reaping attack had a multiplier of some kind, instead of being magical. I don't remember what kind. For what it's worth, I believe both times I had the witch's knife.
FAKEEDIT: Oh wait a second, both times the base damage of the reaping attack was two. Maybe that has something to do with it.