So I completed Halo 1-4 on Legendary through the Master Chief Collection (not including ODST), and I got the achievement “Legend”, but the Helioskrill armour is still locked for me. Anyone know what I can do to solve this issue?
Take a look at this thread: Help! I didn’t get my Helioskrill!.
It should hopefully solve the issue you have.
Please follow the link in the post above.