what's up mothershuckers
okay so for Xerneas, it'll be called Breath of Life, and essentially what it does is that whenever Xerneas either uses a healing move or has a healing move used on it, it and the other involved get attack and special attack boosts.
For Yveltal, it'll be Verge of Death, which essentially means that every attack done against it, not based on how much hp it takes but simply how many hits, until fainting, one of Yveltal's stats will boost randomly. Essentially "the pain makes me stronger!!!!1!!11!!"