Moonshine Brine is one of three upgrade materials acquired via completing Abyssos (Savage) raids in FFXIV. One can be purchased in exchange for four total copies of Abyssos Mythos III — a type of currency dropped up completing Abyssos: The Seventh Circle (Savage). The brine can be used to upgrade Lunar Envoy weapons.
To make the purchase, players must speak to either Mylenie in Labyrinthos (X: 8.3, Y: 27.6) or Djole in Radz-at-Han (X: 10.3, Y: 9.6). For reference, Mylenie is just east of the Aporia Aetheryte (inside the facility where you start the Pandaemonium raid series). Whereas Djole operates in the West Balshahn Bazaar. Select any option marked “Abyssos Mythos Exchange” and scroll to the bottom of the available purchases. Moonshine Brine is the third purchase option from the bottom.
Moonshine Brine is also a loot drop from the Abyssos: The Seventh Circle (Savage) raid itself. Though players must compete for it by rolling Need or Greed.
Once you have the brine, speak to the NPC Khaldeen in Radz-at-Han (X: 10.8, Y: 10.4). They are also located in the West Balshahn Bazaar — next to the Tomestone Exchange merchant Cihanti.
Choose any of the “Lunar Envoy’s Gear Augmentation” dialogue options (DoW I, DoW II, or DoM) to use the brine to upgrade a particular weapon. You will of course also need at least one Lunar Envoy weapon (which can be purchased from Cihanti in exchange for tomestones). No other currency is required to make the purchase.
The new Augmented Lunar Envoy weapon will come in at Item Level 630 — as opposed to Item Level 620 on the normal gear.
The other Lunar Envoy augmentation materials include: