Minecraft Dungeons:Soul Sand Valley

The soul sand valley is a secret mission $$, in the Flames of the Nether DLC. It is unlocked through a secret that can generate in the Crimson Forest, which in turn can be unlocked from a secret in the Warped Forest.

Story
"DARK DUNES – The Soul Sand Valley is a bone-chilling part of the Nether that even the boldest of warriors dread to tread. Gather your wits and hold your weapon close as you cross the shifting sands."

- In-game story

"Deep within the twisted wilds of the Nether is a bone-chilling place when even heroes hesitate to go: The Soul Sand Valley. Will you be able to safely navigate the haunted sands?"

- Opening Cinematic

Trivia

 * It is currently the only mission that is unlocked inside another secret mission.
 * It is the only Flames of the Nether mission that contains regular skeletons and wraiths.
 * Unlike that in the base game, the speed of heroes doesn't seem to reduce when walking on soul soil or soul sand and they don't seem to change the orange flames made by blazes into blue soul fires.