The Haunted Manor dungeon was added in the Halloween update of 2019. It contains the most individual boss fights in any dungeon since and a special secondary final boss that you have to get lucky and find entry keys to fight.
Normal Spawns in the darkly lit Building
Spectres - Like Vexes they can sometimes fly and phase through walls, but are much stronger.
Attic Keepers - You'll have to defeat 3 of these Ghast creatures in the attic of the manor to finish the dungeon and be able to fight The Headless Horseman.
Ghost Chef - A miniboss that will fling harmful exploding cakes at you and holds the Upstairs Key. Can drop the key and on occasion, a Cake Cannon
Spider Queen - A dangerous queen that hatches exploding spider children. Guards a chest and will occasionally drop a Death's Diamond.
Headless Horseman - A difficult final boss riding a skeleton horse that is fought atop the Manor.
The Reaper - One of the Raid Bosses to this dungeon, you'll need to collect "Deaths Diamonds" and probably bring a whole team to fight him.
The Devil - The hardest Raid Boss of the Haunted Mansion. You'll need 20 Quest Points and a Death's Skull to even fight him.
You'll need around 17500 Money and a few quest points to do the main dungeon. Once you take it though, the Manor Keeper will let you in. You will then have to kill the Ghost Chef to take the Upstairs Key from him. Along the way you can explore to find some chests that contain goodies like Diamonds, Iron, Gold, sometimes the Spooky Chest and entry keys for other Manor bosses. Once you defeat the Ghost Chef and take his key, you can bring it to the Manor Keeper and he'll let you acces the upstairs area, therefore advancing in the Dungeon. Here you can fight the Spider Queen, an optional miniboss that can give you Diamonds, Poison Webs, Cobwebs, String, and maybe a Spooky Chest. There are also multiple dungeon chests on that floor too. Continuing up the stairs you'll find the Attic and its keepers, the Attic Keepers. They mainly fly around and sometimes blasts you with a series of explosions. They aren't too harmful but they can knock you down one of the many holes in the floor. Some of those holes even leads to the ground floor where you started, and you'll have to pay a fee to get back up. Once you kill 3 you can talk to the Manor Keeper to reach the roof and fight the Headless Horseman.
Manor Mobs Challenge
The Manor Mobs Challenge Quest requires you to beat the Haunted Mansion at least once before you can take it. 15 Specters, 6 Attic Keepers, and 10 Evil Pumpkins need to be killed in that order to complete the quest. Spectres are found on the first/ground and second floor. Attic Keepers are found in the Attic mostly but rarely you'll see one fly down to the second floor. Evil Pumpkins are only found on the roof where you fight the Headless Horseman. When you go to fight the Headless Horseman you cannot go back down without killing him or dying, so make sure you have all the other objectives done first.