Once you complete the Battle Pet Tamers for Northrend, you’ll be eligible to pick up the next quest for Cataclysm.

There are three Master Pet Tamers to defeat and may be tackled in any order.

Brok
Brok
<Master Pet Tamer>
Location – Mount Hyjal, by the road to Nordrassil
Level 25 Pets: Ashtail (Beast), Kali (Magic) and Incinderous (Critter)

Bordin Steadyfist
Bordin Stradyfist
<Master Pet Tamer>
Location – Deepholm, just outside the Temple of Earth
Level 25 Pets: Crystallas (Critter), Ruby and Fracture (2x Elementals)

Goz Banefury
Goz Banefury
<Master Pet Tamer>
Location – The Devouring Breach in the Twilight Highlands
Level 25 Pets: – Amythel (Magic), Helios (Beast) and Twilight (Elemental)

Defeating these three will open up the quest for the Grand Master of Cataclysm.

Obalis
Obalis
<Grand Master Pet Tamer>
Location – Uldum, top of Vir’naal Dam
Level 25 Epic Pets: Pyth (Beast), Spring (Flying) and Clatter (Critter)