Aberrant Wurm

Trample.
Whenever Aberrant Wurm blocks or becomes blocked by a creature, you may pay {1}{b}: put a -1/-1 counter on the blocking creature and a +1/+1 counter on Aberrant Wurm.
At the beginning of your upkeep, put a -1/-1 counter on Aberrant Wurm.
He needs to swallow things.