There are numerous quests in No Man's Sky, each one giving the player specific goals and offering powerful upgrades and rewards. "They Who Returned" is one such quest that explores the game's lore.
How to start the autophage mission They Who Returned – No Man's Sky Echoes Update – NMS Scottish Rod
They Who Returned was added with the Echoes update and offers a glimpse at a new alien race called Autophages. Most importantly, players who complete this quest will be able to wield a futuristic staff instead of a multi-tool.
Here's a full walkthrough of the They Who Returned quest in NMS.
Since They Who Returned is an endgame quest, you can’t jump right into the mission. You’ll need to complete two main objectives to unlock the quest: