The chest is hidden from view by a short, broken piece of wall.
From any Vulgrim location, enter the Serpent Hole for Dry Road. From the start point, simply turn around to see the chest. Behind you, believe it!
The artifact is in plain sight, on the edge of the cliff. While you're out here, don't forget to glide over to the Vulgrim location. You can make it across the canyon with a precise Shadowglide from the scruff of Demonic Growth.
The Crossroads / Scalding Gallow / Choking Grounds and The Broken Stair / Twilight Cathedral / Drowned Pass / Anvil's Forge / The Hollows / Dry Road / The Ashlands / Iron Canopy / The Black Throne and Eden