Legend (Halo: The Master Chief Collection achievement)
From Halopedia, the Halo wiki
Legend is an achievement in Halo: The Master Chief Collection. It is unlocked by completing all campaigns on Legendary difficulty. The achievement is worth 50 gamerscore and will unlock the HELIOSKRILL armor in Halo 5: Guardians. Despite the achievement description, it does not require the completion of the campaigns of Halo: Reach or Halo 3: ODST, because the achievement was created at the initial release of the collection, at the time of which the two games were not yet added to the collection.