Quote Originally Posted by Corviss the Lich View Post
You need the Scroll which can be obtained by completing the event quest.

If you already have it and still have trouble crafting, you need to click the "(Arcane, Mythic, Legend, Epic) Egg" and you'll be able to choose what pet you want to craft from your inventory as long as you have the rarity needed.
This is correct - you need to specify which of your Eggs you wish to add to the recipe before crafting a Companion Essence.