From Halopedia, the Halo wiki
Crackin' Skulls is an achievement for Halo: Reach multiplayer matchmaking added with the Defiant Map Pack. The achievement was unlocked 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 could be earned on any multiplayer map.
As with Candy from a Baby and Return to Sender, this achievement did not actually require the player to be playing matchmaking at all to earn it; it was possible to unlock the achievement while playing custom games, as long as the gametype being played was electronically signed by Bungie. (That is, if the gametype was the same one used in matchmaking.) However, this method required downloading the specific gametype file; with those services shut down, unlocking the achievement in a custom game is now functionally impossible.