This is the location of Ferals' Den (Image via Diablo 4) The game offers many Legendary Aspects for you to obtain, and Aspect of Quicksand can be acquired by completing a dungeon named Ferals’ Den.

The aspect of Quicksand in Diablo 4 slows down the enemy hit using the damaging Earth skills for a short period. This aspect is exclusive to the Druid class and enhances the effects of earth skills.