You need to complete the following achievements:
Aquatic Acquiescence
Mousing Around
Ragnaros, Watch and Learn
Murlocs, Harpies, and Wolvar, Oh My!
Roboteer
Best of the Beasts
Dragons!
Flock Together
Master of Magic
The Lil' Necromancer
You will receive Nightmare Treant pet
Here you can buy Family Familiar achievement for EU or US Servers.
You'll get guide for completing Family Familiar achievement
