Legendary All Skulls On (LASO), also known as Mythic, is a fan-created difficulty that requires setting the difficulty to Legendary and activating all skulls. Solo LASO (SLASO) is LASO with only one player. The Iron skull forces the player to complete the mission with zero deaths or saves. LASO is possible in all Halo first-person shooters post-Halo: Combat Evolved since skulls first appeared in Halo 2. However, a LASO run is possible in Halo: Combat Evolved Anniversary and in its entry in Halo: The Master Chief Collection since skulls were introduced to the game for those re-releases.
- Completing Campaign missions on LASO has been featured as challenges in Halo: Reach and Halo 4 and Achievements in Halo: The Master Chief Collection.
- In Halo 4, completing all Campaign missions on LASO difficulty will unlock the LASO emblem.