Savrri, the Golden

Immortal (Spells and abilities cannot reduce this
creature's toughness less than 1)
Whenever another creature enters the battlefield
under your control you may put a +1/+1 counter
on it or Savrri, the Golden.
When Savrri, the Golden has 5 or more +1/+1 counters on it, exile her and return her
transformed.