Masters of the Plague

Cumulative upkeep {b} (At the beginning of your
upkeep, if this permanent is on the battlefield, put
an age counter on this permanent. Then you may
pay {b} for each age counter on it. If you don't,
sacrifice it.)
For each age counter on this enchantment,
all zombies you control gain +X/+X. These
counters are removed during your endstep.