Burden of Eternity

Burdens of Eternity are used to upgrade any bind to account item level 496 timeless armor token into a soulbound item level 535 randomly-enchanted item. Note that this will not work on Timeless Curio.

Source
Rarely found on the corpse of any mob or in any chest on Timeless Isle, or purchased for from.

Additionally, the one-time quest The Archiereus Of Flame awards one, and the Blazing Chest found on the west side of Ordon Sanctuary is guaranteed to contain one as well.

Beyond that, any of the Timeless Chests in Kukuru's Grotto have a slim chance to contain one as well. Each cache requires a Kukuru's Cache Key, available for purchase for from Master Kukuru.