So I'm working I grimgrin and someone was talking about using {gravecrawler} to combo out or something about GG swinging for 100/100. Can anyone expand on this please
{grim-grin corpse born} + {grave crawler} + {Rooftop storm}
Because you can return {Gravecrawler} to the battlefield for {B} if you control a zombie, which you do since {Grimgrin, Corpse-Born} is a zombie, you can activate Grimgrin's "sacrifice another creature to put a +1/+1 counter on Grimgrin" ability X times, where X is the amount of black mana you have available.
Blue/black has multiple ways to produce infinite black mana through simple combos(Edit: or {Rooftop Storm} as spencer posted to negate the cost entirely), which if done effectively makes Grimgrin an infinity/infinity, capable of easily killing a player in one blow.