Creatures with power less than Champion of Lambholt's power can't block creatures you control.
Whenever another creature enters the battlefield under your control, put a +1/+1 counter on Champion of Lambholt.
5/1/2012 Creatures with power less than Champion of Lambholt can’t block any creature you control, not just Champion of Lambholt.
5/1/2012 Champion of Lambholt’s ability works even if it isn’t attacking.
5/1/2012 The comparison of power is only done when blockers are declared. Increasing the power of a blocking creature (or decreasing the power of Champion of Lambholt) after this point won’t cause any creature to stop blocking or become unblocked.