"Atsu's Torch! It was lost in this place... if we carry it to the Shrouded Lantern, it might show us the way."
The Shrouded Lantern is contained within the Misty Woods, and is the source of the mist. It is normally lit, but after Atsu's Torch faded, the mists returned. This also caused Tatsu to lose her way in the mists.
The only known way to light the lantern is by returning Atsu's Torch to the lantern. Doing so will reveal the Gumon Seal. After the lantern is lit, the mists are cleared and the woods stop shifting. However, this results in only a portion of the woods being accessible. In the Definitive Edition, Ori can extinguish and relight the lantern at will after bringing Atsu's Torch to it. Extinguishing the lantern restores the mists to the woods and allows them to shift again, granting access to the entire area once more.