Test of Fortitude

Hexproof
{x}{p/g}{p/r}{p/u}: Up to X target creatures deal damage to themselves equal to their power. Then put an Experience counter on this card for each creature destroyed by this effect. Anyone may activate this
effect only as a sorcery.
If this card has 10 or more Experience counters on it,
this card becomes a 3/6 Elemental Legendary
Creature with Flying, Trample, and Double Strike
in addition to its other types and abilities.