Mythic.
From Halopedia, the Halo wiki
Mythic is a level difficulty from Halo Zero. It is the next in line after Legendary, and is only available by defeating Halo Zero's campaign on Easy, Normal, Heroic, and Legendary. If you defeat Halo Zero's campaign on Mythic, you will gain the ability to pick up the gold (commander) Elite's energy sword.
Note: This is a difficulty level in Halo Zero and is not to be confused with the skulls Mythic (Halo 2), and Mythic (Halo 3).