In order to get into UBRS and clear to the rookery whelps you need to Seal of Ascension. The Seal of Ascension can be obtained from looting the item Unadorned Seal of Ascension (random drop from Lower Blackrock Spire) and completing the quest chain. Make your way to Pyroguard Emberseer, kill him with at least 3 people (for summoning) and he will open the door to the rookery whelps. Once there have your tank round up all the whelps then when they are all at the party AoE them down, the 15 second timer starts when the first person does damage so this is the easiest method.