Auron’s Celestial Weapon, the Masamune, is acquired in two parts. For the first portion, you will need to have progressed the story enough to where you have access to the Cavern of the Stolen Fayth, which is right before Mount Gagazet. Go down to the area with the Cavern’s entrance, but don’t go inside. Instead, there is another path in the southeastern corner of this screen, which will lead to a dead end. On the left side, along this path, you will find a sword stuck in the ground, between two rocks. That is the Rusty Sword, so interact with it to add it to your inventory.
For the next bit, find your way to the Djose Highroad and head backwards, to the camp that was used for Operation Mi’ihen. Use the crates there to get to the northern end of the Mushroom Rock Road. Follow it until you can go north on the first path (you were previously blocked from this section during the main story section), where you should see a water jet that launches you to a new area. There will be a statue of Lord Mi’ihen here and if you interact with it, you will be able to use the Rusty Sword. Doing this will make a glyph appear on the nearby wall, so touch the glyph and you will be able to get the Masamune.
Fully powered up, the abilities for the Masamune are:
- Break Damage Limit
- Triple Overdrive
- First Strike
The Mars Crest can be found on the Mi’ihen Highroad, in the Oldroad section of that area. If you fought the Chocobo Eater and got pushed off by the boss, then you only need to head south to find the chest with the crest at the dead end. If you didn’t get pushed off the cliff by the Chocobo Eater, then head north from the Travel Agency until you get to a fork. Take the southeastern path to arrive on the Oldroad and at the very end, you will find the Mars Crest.
To obtain the Mars Sigil for Auron, you will need to unlock a total of ten Area/Species Creations in the Monster Arena. Area Creations are unlocked upon capturing at least one of each fiend in an area, while Species Creations require a certain number for a species. Once you have done this, the Mars Sigil will be in the chest in front of the Monster Arena NPC.