Slender Man-

While not specifically a stand-alone dungeon, most of the event takes place within a special area created just for the event, The Slender Woods! This event was created in tribute to the Slenderman urban legend that began with Victor Surge (real name: Eric Knudsen) posted a pair of doctored photos online that were said to have been discovered in a small town library. In the images, dark figure, overly tall and thin, is standing behind children, blissfully unaware of the presence.
Players must decipher the map they discover and travel to the Slender Woods, retrieve all the notes about the Slender Man and escape before it catches them. This includes the first-ever "cut scene" on The Expanse, where players must watch the end events unfold without being able to move or teleport.

Check out the short video of the event on YouTube:

Slender Man
(Thumbnail caption: Players must defeat a certain number of enemies to advance into the dungeon.)