The Hyrule Garrison Ruins are a location from The Legend of Zelda: Breath of the Wild. It is located in the area southeast of the Central Tower and north of Windvane Meadow in the Hyrule Field region. It was the sight of a garrison before the Great Calamity before becoming overrun by Guardian Stalkers two of which still patrol the Garrison one hundred years later. Ironically the Central Tower was buried where the Hyrule Garrison constructed. An Opal, Forest Dweller's Sword, and Serpentine Spear can be found within treasure chests left behind in the ruins.
The Master Trials
Upon "The Master Trials" DLC pack installation and during the side quest "EX Treasure: Phantasma", Link can find a Treasure Chest containing the Phantom Greaves and buried in the southeast of the Hyrule Garrison Ruins.