A magical lantern at Gulpax's Workshop used for interplanar travel.

Searching through scrolls in the Temple of Shintal, you find an interesting sketch.

Someone has also scribbled a few notes: “Lantern of Wayward Souls. Gulpax’s Workshop. Secure at all costs.” - Festival of Heroes (GS-ADV1)

The Immortal Knight gives you and Nayo a seemingly difficult task.

“Travel together. Find and open Kethra’s Door. Gain her help in retrieving the lantern from the Order of Assassins. Until you do, none of us are safe.” - Imbri - The Hive (GS-ADV3)

While exploring the lands near Redtooth Peak and Goldbelly Mountain, you are caught by surprise by a would be assassin. A wiry Hobgoblin appears seemingly out of nowhere brandishing a dagger in one hand while holding the Lantern of Wayward Souls in the other. Before you can react, he is gone in a flash. - (GS-Guide- ADV4-26)

You find a hand drawn picture of the lantern in Gulpax's journal.