The Emerald Key is the key which is needed to open the gates to The Spire in the Temple of Cenarius.
The quest chain starts with a "strange crystal" which drops from General Exirios in the Green Terrace. It's a 100% drop chance for those who haven't already got it, and everyone can loot it. The quests asks you to speak with Ysera who again directs you to the leader of The Renewed. She tells you that only through a specific ritual will you be able to make the key. You will then get two separate quest chains, one which certainly is a lot more challenging than the other. The less challenging is to make the key itself, which is just basically finding raw materials which can be crafted, bought at the auction house or looted from specific mobs all over the Emerald Dream. The other quest chain is more difficult since it involves getting relics from the hardest heroics in the game, except the Rift of Aln dungeons since they won't be introduced that early in the expansion. To do the ritual players will need certain relics with powers from the Ancients who battled prior to the Sundering. You are being directed to G'Hanir in the north. In Dreamtalon Ascension after you've defeated Aviana and she flies away, a feather will fall down which you can pick up. Next part is in Agamaggus, and when you free Agamaggan from Qye'Luhan's grasp he gives you a tusk shard. You are then sent to Fernskin Den to save Ursoc and Ursol and by talking to them you will aquire a tooth shard from Ursoc and a claw shard from Ursol. The Synagouge in Felflame Palace is your next destination where you save Goldrinn from the Emerald Flame and he will give you a ball of fur. The last part takes place in Tarhaqi Grounds. After freeing Tortollus he will give you a shell shard. With the last part the ritual begins and you have been given the Emerald Key and the access to the Spire.