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It is said (by some elder sage or other) that the hatchlings are highly susceptible, and that each essence exerts a unique influence on the incubating creature. Is there method to this madness? Trial and error, and some comparison of results, will surely reveal the rules of Greevil husbandry.
this basically means that different essences will affect the outcome of the egg when it hatches, this is mainly a thread to try and gather information about the different possible outcomes and how to get them.
first things first....

Each Egg has 15 slots for essences :
3 red slots (Exort essence)
3 Blue slots (Quas Essence)
3 Yellow slots (Wex Essence)
2 Green slots (Subtle Essence)
2 Orange slots (Strange Essence
2 purple slots (Unusual Essence)

Different Types of essences :
Quas Essence
Wex Essence
Exort Essence
Unusual Essence
Strange Essence
Subtle Essence
Shadow Essence

WARNING : for those of you who are willing to experiment, DON'T use a shadow essence on an egg that already contains other types of essences, it will always convert into a dreaded egg (Black) no matter what, and your essences already put into the egg will be kinda useless, since you can just put a shadow essence into an empty egg to get the same result
Different Egg outcomes according to essences feeded : (Image can be found Here, Thanks dspir1t.)
The difference in level between courier types results in different animations for them.

Red Eggs : (Majority of Red Essences)
Ferocious Egg (Red type 1) : TBD, Hatch outcome : Red Greevil Level 1
Brutal Egg (Red type 2) : 2 Exort Essence, Hatch outcome : Red Greevil Level 2
Merciless Egg (Red type 3) : 3 Exort Essence, Hatch outcome : Red Greevil Level 3
Yellow Eggs : (Majority of Yellow Essences)
Anxious Egg (Yellow type 1) : TBD, Hatch outcome : Yellow Greevil Level 1
Eager Egg (Yellow type 2) : 2 Wex Essence, Hatch outcome : Yellow Greevil Level 2
Spirited Egg (Yellow type 3) : 3 Wex Essence, Hatch outcome : Yellow Greevil Level 3
Blue Eggs : (Majority of Blue Essences)
Stubborn Egg (Blue type 1) : TBD, Hatch outcome : Blue Greevil Level 1
Resolute Egg (Blue type 2) : 2 Quas Essence, , Hatch outcome : Blue Greevil Level 2
Determined Egg (Blue type 3) : 3 Quas Essence, Hatch outcome : Blue Greevil Level 3
Green Eggs : (Majority of Blue/Yellow Essences)
Covetous Egg (Green type 1) : 2 Quas/ 2 Wex, Hatch outcome : Green Greevil Level 2
Greedy Egg (Green type 2) : 3 Quas/ 3 Wex, Hatch outcome : Green Greevil Level 3
Orange Eggs : (Majority of Red/Yellow Essences)
Ravenous Egg (Orange type 1) : 2 Exort / 2 Wex, Hatch outcome : Orange Greevil Level 2
Voracious Egg (Orange type 2) : 3 Exort / 3 Wex, Hatch outcome : Orange Greevil Level 3
Purple Eggs : (Majority of Blue/Red Essences)
Cruel Egg (Purple type 1) : 2 Exort / 2 Quas, Hatch outcome : Purple Greevil Level 2
Wicked Egg (Purple type 2) : 3 Exort / 3 Quas, Hatch outcome : Purple Greevil Level 3
White Eggs (Majority of Red/Yellow/Blue Essences)
Blessed Egg (White type 1) : 2 Quas/2 Wex/2 Exort, Hatch outcome : White Greevil Level 2 (needs confirmation)
Seraphic Egg (White type 2) : 3 Quas/3 Wex/ 3 Exort, Hatch outcome : White Greevil Level 3 (Source, Confirmed Thanks Treguard)

Black Eggs
Dreaded Egg (Black type 1) : 1 Shadow Essence, Hatch outcome : Black Greevil Level 0

Natural Eggs
Imbued Egg (Natural type 1): 1 essence of any type except shadow OR All other combinations, Hatch outcome : Normal Greevil Level 1 (Naked)

SPECULATIONS : (Please confirm these or deny them if you got any proof)
Unusual essence will not change the egg color or the outcome, but it might cause the courier to be unusual??
IF the speculation about unusual essence is correct, will strange essence cause the courier to be strange?
Adding a shadow essence into an egg will always turn it into a dreaded egg no matter what type of essences you had before, or even if it was empty, and it prevents you from adding more essences (Source, Confirmed thanks shepherdlui.)
Eggs will hatch after the event is over, and not anytime before.
Egg outcomes will be Greevil Couriers, The Greevil will have a number of different parts ranging from Ears, Eyes, Hair, Horns, Nose, Tails, Teeth and Wings. He will also come in a range of different colours etc. The combination you get will be determined from whatever essences you feed your egg. (Source, Confirmed Thanks Cyborgmatt.)
lets try to make this post as informative as possible so people can get the most out of this event within its duration. (and try to get as many people contributing to this, the more input the faster this gets completed and confirmed)

Happy Diretide.