Ouroubouroak

Protection from green, convoke
At the beginning of your end step, move the bottom card in your graveyard to the bottom of your library. If there are no cards in your graveyard, transform Ourobouroak instead.
{t}, {g}{g}{g}{x}{x}: Move the bottom X cards in your graveyard to the bottom of your library. This ability has convoke. (Your creatures can help activate this ability. Each creature you tap while activating this ability pays for {1} or one mana of that creature's color.)
This mystical loop of vines recycles any waste which passes through it. But it has a sinister side - if not fed, it becomes dangerously proactive...