If the essences are combined within a rare gemstone such as a Southfury moonstone, you'll be able to release them when you're prepared to challenge Anzu.
The only problem is that Southfury moonstones are only found on the Azshara side of the river, on the cliffs near the river's source.
Tragedy befell the last night elf party that sought the stone. Their boat is still anchored in the river near where they sighted the moonstone. Keep your eyes open and approach the site by land from the Azshara side.