Read the description on the Sythe. It suggests that you remember nothing from the battle. Thus, you lose all experience whenever you use the Sythe. Although, if you have XP bonuses it doesn't remove all the XP then, it is a simple subtraction.

no i don't think so. it's a personal XP modifier. the way it seems to work is that first, XP is distributed among champions & troops, and then the modifiers kick in, such as XP traits, tutelage and items. in this case, the item reduces the XP for the guy that uses it, but it doesn't affect XP for the other survivors of the fight.