Runed Girdle is a quest item. It is 1 of 3 items required to complete the quest Secrets in the Dark.
Item acquisition
Runed Girdle has restrictions on where or how it can drop.
Dropped in the Red Vale after surviving an Obelisk of Rust encounter.
Runed Girdle may be dropped from an Obelisk of Rust found in the Red Vale[guesswork?]