Halo 3's unofficial achievement.
If you've already completed Halo 3's campaign on Legendary difficulty, you won't earn any additional gamerscore or achievements for slugging your way through a solo run with all skulls maniacally turned on, but you will earn mad street cred and a heightened sense of self worth. If you just read that sentence and shuddered a bit at the prospect of such a daunting task, fear not, The Tyrant over at HBO is meticulously stitching together a level-by-level guide, offering his gentle words, and more recently, embedded videos to hold your hand throughout your attempt. He's dubbed the run "Mythic" difficulty.
Halo 3 Mythic Walkthrough