With the patch 8.2 Azeroth opened the gate of a new pet battle dungeon: Stratholme.
To start this journey head to Radek Fuselock for the horde side or Tizzy Gearjolt if you're from the alliance.
Reports of sinister machinations lead us to Sean Wilkers, a paladin who keeps company to us during this expedition and later will function as this dungeon's pet vendor.

Your first playthrough grants an Ultimate Battle-Training Stone as usual, and the following runs in the Challenge Mode reward the new currency Cleansed Remains which is used to buy pets. Note that these pets are not tradable, so you have to work for them.



Furthermore as a novelty to pet battle dungeons there are two special bosses within Stratholme: Fras Siabi and Postmaster Malown.

1. Fras Siabi

works like a rare boss in the form of a world quest, meaning that there is a low chance for him to spawn randomly. He appears in stage 7 of the dungeon, right behind the Risen Guard and beating him awards an additional Cleansed Remains. However if you are really eager to get this currency you can leave the dungeon prematurely and progress to stage 7 again until he spawns. But be warned: he is a tough opponent!




To fight

2. Postmaster Malown

you first have to make him spawn. In order to do that find and right-click a letter that is laying hidden inside the dungeon. Once you've done that go the mailbox at 57.8 13.6 (in the vicinity of Nefarious Terry) and he'll appear. Now you have 20 minutes to fight him. After that time Malown will disappear. Beating him rewards you with the achievement Malowned and the pet 燃盡.

There are multiple points where the letter can spawn, though it's not guaranteed it will be there.
If you are using TomTom you can copy those coordinates to find the letter:

/way 68.9 54.0 Letter
/way 59.7 40.6 Letter
/way 56.9 50.8 Letter
/way 57.8 36.9 Letter
/way 40.2 10.4 Letter
/way 48.6 24.9 Letter
/way 57.8 13.6 Mailbox