Not entirely sure if this is in the forum somewhere, had a good stroll and couldn’t find it so I’m making a new one 
Mythic items, the ones that have the 3 star banner across them; stances, one time use weapons, vehicles and armour etc, are these obtainable threw packs or do you have to do special things like max out a commedation or complete previous halo games on Legendary etc?
If anyone has any insight to this please share, Thanks
They majority of the mythic items are attainable through req packs. However, the Nightfall armor, Mark VI (Gen 1), Mark VI (scarred), and Helioskrill armor are acquired through activities done outside of the game.
What do you have to do to get them?
Helioskrill Armor:
Complete the Legend achievement in Halo: Master Chief Collection by completing all the MCC campaigns on the legendary difficulty
Nightfall Armor:
Watch all five episodes of the Halo: Nightfall series on the Halo Channel
Mark VI Armor:
Earn the Cairo Station achievement in Halo: Master Chief Collection by completing the Cairo Station mission in Halo 2
BTW keep in mind that the armor may not unlock immediately after you complete these activities. It can take some time.