This is a repost of Valve's official event page here
'Tis the Season of the Dire
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The tide of Radiance has sunk into it's lowest ebb, long summer days have give way to longer nights. The river runs red and the woods run wild with the grief-loving imps known as greevils. Prepare yourself for a new DOTA 2 game mode fulll of mysterious eggs...rattling coffins...and horrible candy...what's not to love?
The most ominous event of Diretide is the annual suspension of Roshan's enchantment. Freed from his fearful den, he is compelled to wander in search of the one food that can satisfy his infernal craving: Greevil Taffy!
While Roshan roams through endless night, heroes fill their coffers with sweet delight, each team racing to collect more taffy than the other...until Roshan catches their scent! And then, like a scaly, winged, downright epic version of the neighborhood bully, Roshan will let no puny heroes stand between him and his taffy.
Once he has stuffed himself, the affliction known as Sugar Rush comes over Roshan--and then the whole neighborhood is in trouble. It will take the combined strength, speed and cunning of both Radiant and Dire to bring Roshan down.
After you trounce the beast, the rewards are sweet indeed. For where normally he harbors cheese and immortality, in the Diretide season Roshan drops rare essences and even (weirdly enough) eggs!*
Trick or Teeth
When it comes to making Greevil Taffy, the Confectioners of Yhauron have guarded their secret process closely, and no wonder. It is a truly horrible process!
In fact, should Greevils ever discover what has been done in their name, no Yhauronish candymaker could ever know a moment's peace. But we can spill the secrets here, because Greevils never read (and Yhauron is not pronounced anything like the way it is spelled).
It's all very simple really. Take a Greevil. Stretch and fold it ten thousand times on a colorful greevil-taffy-torture-rack, and abracadabra: Colorful, savory, tough and extremely gristly taffy!
Is it any wonder that only Roshan can stomach the stuff?
Fortunately for You
If you are a gentle soul who finds the Diretide season too terrifying, you can exorcise the spirits of Direness by shopping. At the Dota Store you will find seasonal enhancements for Invoker, Death Prophet, Windrunner and Antimage, along with keys for opening the especially Dire crates that drop during Diretide battles. These are all part of the lamentable Dire celebration, so they are possibly a bit frightening. But there is nothing the least bit ominous about an adorable baby Roshan.
Essences & Eggs
The culmination of the Diretide festival, ironically enough, is an occasion of joy.
Joy, that is, if you find hope in the arrival of swarms of chaos-loving Greevils, for this is when they Hatch!
Throughout the Diretide celebrations, you must treat your egg well. Feed it the essences dropped by Roshan...or don't! It's really up to you. 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.
To every hero who stalks and slays Roshan, an egg will surely fall. Should you find yourself wishing to experiment further, Greevil eggs are also available in the Dota Store. Buy a dozen--or even a gross, in honor of the flavor of Greevil Taffy.