Yggdrasil, the One Tree

Yggdrasil can be your commander. It's green.
At the beginning of your upkeep, if Yggdrasil is
in your command zone, you may put it onto the battlefield.
{t}: Reveal the top card of your library. If it's a creature card with a lower converted mana cost than the number of lands you control, you may cast it without paying its mana cost. If you do, exile Yggdrasil.