Devethi Treewarden

As long as you control a Forest, Devethi
Treewarden has +1/+0 and "Whenever Devethi Treewarden blocks or is blocked by another
creature, tap that creature. It doesn't untap
during its controller's next untap step."
{1}{g/u}, {t}: Put a +1/+1 counter on a green
creature you control.