In order to reach the Mad King's Realm, which is where the Labyrinth is located, you will need to go through one of the Haunted Doors in Gendarran Fields. (You may be able to go through any Haunted Door but I have not confirmed this yet) To reach the Mad King's Labyrinth you will need to take the portal up on the raised platform as you come in. It's a little circular thing on the ground, extremely easy to see. Upon taking this portal you will be in an extremely large maze filled with high level dangerous Halloween mobs.
These Halloween mobs have the same effects as the Risen mobs in Malchor's Leap in Cursed Shore. Some Cripple, some knock you down, other apply ailments on you, cure ailments from each other etc etc. This maze can be ungodly annoying to navigate is what I am trying to say. Luckily a lot of other people are navigating it too. If you see someone in danger or trouble stop and help them out! Don't be a jerk and just leave their body or leave them to die, this gaming genre doesn't need more jerks so don't add to it!
Below is a picture of the entire Mad King's Labyrinth. I shrunk it a bit so click it to enlarge it.
Throughout this Labyrinth you'll be able to work towards completing multiple different achievements such as "Halloween Huntin'" Which requires you to kill Halloween monsters. Really everything here. Pumpkin Carving which requires you to carve pumpkins found in certain zones throughout the world. There are tons of Pumpkins here for you to carve making this achievement much easier to do. You need 150 of them in all and between here and Lion's Arch it shouldn't be too hard!
But the main point of this guide and the Mad King's Labyrinth is to go over the Champion's that spawn in this area. So far I only know of two Champions, one in the center of Mad King's Labyrinth and one in the southeast. I'm not sure if they spawn elsewhere but these two I've done multiple times. After defeating these champs they will spawn a chest on the ground which has an extremely good chance of containing an Exotic item. You can only do each champion once a day (they reset with dailies) so it's not like you can just endlessly grind them forever.
I'll go into more details later with this guide (hopefully) once I learn more about this area. In the meantime, here is a picture of the general type of loot I get from one of the chests when I complete it: