Now that you have Ziri's gadget, you should have everything you need to take on the Nightmare Amalgamation. Gather as many allies as you can and head back into the Nightmare by way of the portal Bough Shadow in Ashenvale. Find and destroy the Nightmare Amalgamation within its lair. Oh, and one more thing... the beast is in possession of a specific item that is of great interest to me. Bring me whatever you find on its corpse and you'll be well rewarded.