There are various weapons Cloud and company can equip in Final Fantasy VII Remake, and a few can be found by poking your nose about the world. Aerith’s Mythril Rod is one such weapon found out in the wild, but you’ll need to reach the Train Graveyard if you want to grab it.
The Mythril Rod is found early in Chapter 11.
Once you reach the Train Graveyard and kick off Chapter 11 you can pick up the Mythril Rod. Actually, it’s pretty much at the start of the zone.
After leaving the vending machine area and passing through a train you’ll be attacked by a couple wererats. Kill them then cross the train to your left. In the next area look directly ahead to spot another train you can walk through that will lead you back down south.
On the map this will appear to lead you back to the vending machine. Take this path, and round the corner to the left to spot a purple chest. Loot it and the Mythril Rod will be yours to use. This staff is a Magic Attack powerhouse with three unlinked materia slots, so if you want a more offensive Aerith this is your go-to weapon.
The Mythril Rod comes with the Ray of Judgement ability in Final Fantasy VII Remake, a massive burst of energy that hits a target multiple times, and increases the stagger damage bonus on staggered enemies. It will cost you two ATB bars to use, however.
Earning the proficiency bonus is simple enough: use Ray of Judgement on a staggered enemy. Considering it increases the multiplier you will want to do this anyways, but the extra proficiency from the bonus is certainly a plus.