Briostone Trio can be found in the following places:
Starting gear for the Mournstead Infantry
Forsaken Fen: After picking up the Panoptic Ring, you’ll have three paths to progress. Take the path on the left and you’ll find two Briostone Trio behind a rock on the left.
Fitzroy’s Gorge: Dropped by the Mendacious Visage mini-boss.
Upper Calrath B (accessed after completing the "Cistern" location): . In the open area with a minor Skinstealer skulking about that you’ll have to deal with. The small booth on the right is hiding some Unripe Berries and Briostone Trios under breakable objects.
Can be purchased from Gerlinde in Skyrest Bridge by offering the Rune Tablet, which grants access to a new service called Socket Runes along with an expansion of her inventory.
In the area where you have to soulflay the platform that is blocking the ladder in Sunless Skein, head near the tunnel entrance to find some loot. Briostone Trio can be found among some items.
Dropped by Mendacious Visage.
Revelation Depths: After the Umbral Eye of Dieter, notice that there are scaffoldings near your location that you can jump into. Head there and face the Pale Shadow lurking in the area. Also, you will be dealing with Remnants and Miners in this area, thus be careful. After clearing the enemies, pick up the x2 Briostone lying on the ground
Bramis Castle: From the Umbral Flowerbed up the stairs past the Vestige of the Bloody Pilgrim, continue along the path and up the stairs to find a breakable wall. Break the wall and defeat the Ruiner, head to the open archway on the left and go continue along the hall to find another Ruiner patrolling the area. On the left side behind some vases, you can pick up x3 Briostone Trio.
Manse of the Hallowed Brothers: In the part of the Manse Location Walkthrough named Shortcut Galore, Return down to the room with an Emergence Effigy and go back to the Axiom. In this room, jump down through the broken part in the railing and open the gates behind you to open a shortcut. Ahead will be a few Avowed and Radiant Hounds, as well as a Crimson Rector. After defeating them, go through the archway on the right. Behind some piled up wooden crates on the right is a Briostone Trio. On the left, you’ll be able to open a gate that leads back into the courtyard with the doors requiring a key.
Skyrest Bridge: Winterberry sells this item if you complete the quest line "Find the Thief". Completing the quest will trigger Byron and Winterberry to move to Skyrest Bridge and add new wares (including this item) to the merchant’s shop.
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