Deathbringer

Deathbringer costs up to X less to cast where X is
the number of legendary zombies you control.
All other zombies gain deathtouch, and "When
this creature kills another creature, put a death
counter on Deathbringer." All other legendary
zombies also gain first strike and flying.
When Deathbringer has 25 death counters on it,
you win the game.