Sahn'myr, the White Dragon

Vigilance, Immortal (Spells and abilities cannot
reduce this creature's toughness less than 1)
Whenever a white creature enters the battlefield,
put a +1/+1 counter on each white creature you control.
Whenever Sahn'myr, the White Dragon has ten
or more +1/+1 counters on it, exile Sahn'myr,
the White Dragon and return him transformed.