The Heart of the Core

Each of your endsteps, put a mountain counter
on target permanent.
At the end of your upkeep, each creature you
control gets X +!/+1 counters where X is the
number of mountain counters on each of those
creatures.
{5}, {t}: Target land becomes a 0/0 Beast creature
with trample and "When this creature enters the
battlefield, put X +1/+1 counters on it where X
is the number of mountain counters on this
creature.