Ongoing Hearthstone event spawns every hour, cycling through Stormwind, Orgrimmar (well actually just outside Orgrimmar) and Valdrakken. Elite bad guys show up around 6 minutes later but usually a lot of players are waiting and burn them down pretty quickly.
Drops I’ve seen are lot of Hearthstone cards, cosmetic stuff, a useless Toy, interesting mount, and Sarge the mouse.
Finally got Sarge on my 14th battle. Was worth it.
Love the pet, but I also love the toy. It is a hearth toy with a very unique animation.
Guess I never even tried the toy out. All those Hearth toys have always annoyed me as they share same cooldown as your main Hearth so seemed like, why bother?
Now I gotta take a look at the new one … thanks.
Be aware, there are 2 toys from the event.
Stone of the Hearth - drops from boss
Use: Returns you to < Hearthstone Location >. Speak to an Innkeeper in a different place to change your home location.
Hearthstone Game Table - awarded from the Hearthstone Beginner achievement
Use: Place a Hearthstone Game Table and play a worthy opponent!
The transmog drops include:
There is also a mount that can drop from boss as well.
There are 21 cards needed for the Hearthstone Beginner & Hearthstone Card Collection achievements.
If you wave at Sarge, he’ll squeak and then set out a Hearthstone board. He’ll also curl up into a ball and take a nap.
I use an addon called Tome of Teleportation, very handy for all your teleportation items and yes, while all hearths share a cooldown you can setup the addon to use a random hearth each time–a little extra fun for the game.
I like Sarge as well, got lucky and got the mount and pet on the same portal event yesterday.
My friend got Sarge the other day and traded it to me! It is definitely one of the cutest pets I’ve ever collected. I also earned the 1400 pet achievement with him. 
I love my Sarge mount, so I’m glad to have a matching pet.