Rydia's Mist Dragon

Partner with Rydia, Child of Mist (When this
creature enters the battlefield, target player may put
Rydia into their hand from their library, then shuffle.)
Flying
{2}{u}{u}: Until end of turn, the next time a
source would deal damage to Rydia's Mist
Dragon, Rydia's Mist Dragon deals that much
damage to that source's controller.