Kurast Undercity - New Timed Dungeon Coming in Diablo 4: Vessel of Hatred

wowhead.com:

Along with the Dark Citadel, Diablo 4: Vessel of Hatred is bringing with it another new addition — Kurast's Undercity.

This mode takes you through the belly of Kurast, searching for a cure to the plague that scours Nahantu or plundering the many riches left there. Currently, it appears that this will be an activity that sets you on the path to the endgame, either being a leveling or «fresh max level» activity, rather than the current endgame modes we already know.

You will progress through the alleyways of the Undercity, fighting countless hordes of demons while trying to beat the timer, collecting Attunement by defeating enemies, and completing events along the way. Afflicted demons will stand in your way throughout the mode, but slaying these will extend the timer and let you continue your run.

Currently, while we don't have exact specifics on it, theofficial blog release mentions being able to augment future runs and target specific rewards as you need them, while also being able to increase the quality of loot you receive with the Attunement you collect.

Discover the Underbelly of Kurast Undercity

A plague grows deep in the belly of Kurast; a sickness which will devour Nahantu if left unchecked. Search the Undercity for a cure…or plunder its forgotten riches for yourself.

Undercity is an intense new multistage, time-attack dungeon perfect for levelling towards endgame content and targeting powerful items to help you progress. Learn more about the many rewards you can reap as you fight from alley to alley in the belly of the Undercity.

Clear the Streets

In the Undercity, every second counts as you fight against hordes of demons and time itself, as it rapidly depletes out in this time-limited mode. Greatly increase the quality of your rewards by earning Attunement—gained through killing monsters and completing events—slay powerful Afflicted demons to extend your time in the Undercity to reach the final boss.

Augment your Run and Reap the Rewards

Undercity