Gowry the Sage lives deep in the Caelid Wilds, and he’s looking for an Unalloyed Gold Needle to help a girl named Millicent further north in the area. Getting the item he requires is no small task, as it’s in possession of a boss deep in one of Miyazaki’s favorite things: a poisonous swamp.
To find the Unalloyed Gold Needle, you need to defeat the Commander O’Neil boss. Start by heading east from “Astray from Caelid Highway North” Site of Grace, itself south of the Caelid Ruins Site of Grace.
Going east leads to the heart of the Swamp of Aeonia and a large, knotted tree. At the tree’s base is Commander O’Neil, a big knight who summons in additional troops to aid him in battle.
To beat him, clear the mobs and watch out for the huge delay in his attacks. Run away when he starts to swing his halberd around, as he’ll slam it down shortly to either create a small or large AOE that inflicts Scarlet Rot, a powerful poison effect. He’s got a lot of health, so whittling him down will take time, and at about 50%, he’ll summon stronger support.
Killing O’Neil awards the Unalloyed Gold Needle and the Commander’s Standard weapon.
Take the Needle back to Gowry to progress his quest further. He’ll ask for some time to repair the item. Resting at the nearby Site of Grace should progress it, but if not, explore the world for a little while and come back. He’ll award the completed Needle and send you on your way to the north to deliver it to Millicent, its intended recipient.