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.

Trivia

 * As with all the Defiant Map Pack achievements, this achievement does not actually require you 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.