Mjolnir Armories are large crates that you can find all throughout the Halo Infinite Campaign. These crates aren’t meant to benefit Master Chief. They are meant to benefit your spartan in the multiplayer portion of the game. Each Mjolnir Armory contains a multiplayer cosmetic item that you can equip on your spartan. Here is where you can find the Mjolnir Armory in the Outpost Tremonious mission in Halo Infinite.
Outpost Tremonious is the third mission you will play through in the Halo Infinite Campaign. It begins after you defeat Tremonious and take the elevator to the surface of the Zeta Halo Ring. There will be a short section where you need to run through the outpost to get outside. Once you get the door open, a large group of enemies will attack.
Clear out the enemies before searching to make it easier. Once the enemies are cleared out, make your way to the structure that is jutting off of the cliff. You may already be in the location since there are multiple enemies that spawn there. You will find the Mjolnir Armory on the cliff just before the structure. Opening it will get you the Obelisk Stone MKVII Armor Coating.