God Seeker

Defender, Hexproof, Persist.
When Godseeker enters the battlefield, choose a Legendary Creature from any graveyard, put it into its owners hand, then if you control another legendary creature, put a +1/+1 counter on it.

Legendary spells cost {w}{b}{g} less to cast. This effect reduces only the colored mana requirement of that color.

Whenever a legendary creature dies, put a +1/+1 counter on Godseeker. Then if it has 20 or more +1/+1 counters on it, you win the game.