Episode 14.1 Bifrost
The guardian of the Bifrost, bridge between Midgard and Asgard, began losing it's soul to lyrical-like temptations.
As thousands of years passed, she wondered why she used all of her strength to guard the Bifrost. She became confused and chose to seal herself up rather than live in fear.
When she could no longer resist the lyrical temptation a massive crack broke through the bridge connecting Jotunheim and Alfheim, causing the the two worlds to be severed.
The passage through was now nothing but a mysterious fog.
With the origin unknown, no one dared pass through.
Some adventurers have chosen to enter the fog but none have returned. Their fates have likely fallen into the same path as the nameless guardian maiden who locked away her confusion and despair into the dense fog.