How Does The Kurast Undercity Dungeon Work?

The way the Undercity works is:

You’ll proceed through three levels, each with a portal located somewhere within which leads to the next level

You’ll start with 100 seconds and 0 Attunement, and you’ll gain Time (orange) and Attunement (blue) by defeating enemies as you proceed

If the timer hits 0, the run will end and you’ll need to leave and start another run

If you reach the portal in Level 3 before the timer runs out, you’ll be taken to the boss level and the timer will stop

Defeat the boss to claim your rewards and complete the run, with higher levels of Attunement leading to greater rewards

After completing all Undercity missions (listed below), you’ll unlock the ability to enhance your runs for extra challenge and reward

If you are having trouble reaching the end before the timer runs out, simply run past all the enemies and find the portal. Jumping through to the next level gives you extra time, and you are not required to defeat ANY enemies to reach the boss.