Ankh of Eternity

Whenever a creature is destroyed, gain X
energy counter: X is equal to the creature's mana.

When reaching (4+Y) energy counters, flip a coin.
If tails, transform it into Oroboros of Eternal Life.
If heads, transform it into Uroboros of Eternal End.

Every revert reset its energy counter, increasing the
Y energy counter needed by 2: caps at 8.