Rem, the Perfect Waifu

Commander ninjutsu {u}{b} ({u}{b}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand or the command zone tapped and attacking.)
Whenever a Demon you control deals combat damage to a player, reveal the top card of your library and put that card into your hand. Each opponent loses life equal to that card's converted mana cost.
"You took my reason for living and now you want to take my reason for dying?" - Rem