Carrion Grub gets +X/+0, where X is the greatest power among creature cards in your graveyard.
When Carrion Grub enters the battlefield, mill four cards. (Put the top four cards of your library into your graveyard.)
Any counters on creatures that die or effects that affect them until end of turn won't apply to those cards in their owner's graveyard.
Carrion Grub's first ability applies only while it is on the battlefield. In all other zones, its power is 0.