The EX Treasure: Dark Armor is one of DLC Side Quests in The Legend of Zelda: Breath of the Wild. The Quest is located at the Deya Village Ruins, located in the Lake Region to the Southwest of the Dueling Peaks Tower.
Completing this quest will earn you Phantom Ganon’s Armor.
Travel to the flooded ruins of the Deya Village and look for a large destroyed house with its foundation intact – near a broken wagon.
Inside the house is a new journal from Misko the Great Bandit. He’s hidden six more relics from Hyrule Castle. You’ll be given a specific clue for this quest:
“The evil spirit’s armor is hidden within Faron’s sea of trees. Find these pieces at the highest of three waterfalls north of Lake Floria, at the bridge between small waterfalls over Floria River, and at the broken stone bird of Ebara Forest.”
You’ll find all three of these armor pieces in the Faron Region amid the jungle areas.
To find the chest piece, head to the Faron Tower, and glide down to the West towards the Sarjon Woods. Make for the Sarjon Bridge between the two small waterfalls. Jump onto the rocks under the bridge and use Magnesis to pull a chest up containing the Phantom Ganon Armor.
The next clue is a bit tricky – at the highest of three falls North of Lake Floria. This is not in fact the waterfall from which Riola Spring flows – you need to look for the third waterfall in a succession, which would be the one from which Corta Lake flows.
Use the Zora Tunic to swim up the waterfalls until you reach the base of the one leading to Corta Lake, and use Magnesis to search the bottom of the falls to find the chest holding the Phantom Ganon Skull.
The last piece of gear is found in the Ebara Forest, to the East of Lakeside Stable across the long bridge. Along the path, you’ll find three large statues – but the one on the left has a broken head. Search around behind the statue and use Magnesis to spy a chest buried in the rubble with the Phantom Ganon Greaves.
The Phantom Ganon Set is the perfect stealth gear – as it increases stealth – and the set bonus offers a disguise against monster while increasing damage with bone weapons.