Crackin' Skulls is an achievement for Halo: Reach multiplayer matchmaking added with the Defiant Map Pack. The achievement is earned by killing 3 players with the Oddball during an Oddball match. It is represented by a red flaming skull with the number "3" on top of it and an eight-pointed star behind it as a background. The achievement can be earned on any multiplayer map.
As with Candy from a Baby and Return to Sender, this achievement does not actually require the player to be playing matchmaking at all to earn it; It is possible to unlock the achievement while playing custom games, as long as the gametype being played is electronically signed by Bungie. (That is, if the gametype is the same one used in matchmaking.) See here for the specific file.