Unidentified swamp egg

{b}: put a -1/-1 counter on a creature.
When a creature leaves the battlefield put a
permanent strength counter on Unidentified
swamp egg.
When Unidentified swamp egg has 5 or more permanent strength counter on it, exile Unidentified swamp egg remove half the
permanent strength counter rounded down, and
return to the battlefield transformed.
Hatchery(can be a commander with any number
of creatures with hatchery)