Mythic, more commonly referred to as LASO ("Legendary All Skulls On"), SLASO ("Solo Legendary All Skulls On"), CLASO ("Co-Operative Legendary All Skulls On"), or the very extreme version Mythic informally coined by players ("Solo Legendary All Skulls On No Deaths No Save and Quits") is an informal difficulty setting for any Halo title featuring Skulls.
I legit tried talking a friend into doing coop laso, all mcc games, any death resets us back to halo ce. For some reason he absolutely refused. In our laso test run, we only died like 20x on the autumn...
We'd be going a step further too as I am disabled, play one-handed, partially blind, and am using just a mouse as my input. So something like Coop Legendary All Skulls On No Saves MCC Campaign Set (with a) Slightly Crippled.