When it opens players have 10 minutes to enter via. the Adventure Assistant or Khatru Pup, and the entry fee is 20k. There is a limit to how many people can enter, like Dragon Temple, and only level 50+ are able to enter. It is not a PK instance however, and there are four different instances (all of which have identical mobs/rooms). Those instances are Phoenix, Tiger, Dragon, and Tortoise. If you die in the instance you are revived in the same instance at a random corner. If you disconnect, you're kicked out without a refund.
For more information of the Event, you can check out the official Guide: City of Abomiantion - Guide
The mobs drop Astral Orbs, which you can trade for prizes.
The Astral Orbs can be used in order to craft the following items (Beside the retail ones):
At the Dark Messenger inside the Dungeon
The Dungeon will start at the following schedule: