My Rules

Unlike Halo 4, skulls must be collected during the Campaign in order to unlock their use for future playthroughs. Thus, it’s best to wait on attempting this achievement until all 13 skulls have been collected.

When you’re ready, load up any Campaign mission – preferably something shorter such as Alliance – and make sure to turn on at least three skulls in the pre-mission lobby. Any three will do since this mission only lasts ~40 seconds. Now set the difficulty to Heroic, load up the mission, and play it through. Once it has been completed, the achievement will unlock.