The achievement Family Familiar
is the largest and toughest addition to the pet battling world in a long time. It might even beat the Celestial Tournament in required effort!
To complete it you need to earn 10 individual achievements, each asking you to defeat 12 tamers in the Broken Isles with only pets from one specific pet family.
Luckily, when one of the applicable world quests is up, you can fight the tamer repeatedly and thus finish all families in one go, if you have the pets necessary to pull that off.
Once you complete the achievement, you will get your very own tiny, corrupted, adorable Nightmare Treant