[@Sickle-cell]
"Legendary powers:  Being an Avatar representing life itself, Tantalus has the ability to restore others to health, be them pokemon, humans, or even other Avatars, purifying pokemon of the Power Plague, should they be stricken with it. Anything from broken bones to brain haemorrhages are a simple matter, given enough time with the subject. A secondary bonus of his power, is that Tantalus rarely needs to eat, as the power he possesses replaces nutrients gained when eating.  When using his power for a long time or in rapid succession, Tantalus is often forced take a large meal afterwards, one of the only time he can be found eating and which he finds incredibly degrading. The effect is most notable when healing himself several times within a short period - resulting in a spout of rabid binge eating - as he would never even dream of pushing his own limits as hard for anyone else."

Personally, I wouldn't allow this.
Op is the wrong word to use maybe... but I can't think of another one so it's what I'm gunna use; taint that ability, that's what you could have done as Xerneas, but your powers have been dimmed being placed in a human form.
To make it fair I would say that you now have to use YOUR own life force to heal others rather than just going around healing everything like a Mega-jesus.
Admittedly your personality mitigate this somewhat, but still.
Up to the Gm though, just putting forward my thoughts on that.