The great cheese guardian, Roshan, has broken free from the enchantments that would normally confine him to the jungle between Dota 2's lanes and he intends to spend Halloween trick or treating in search of candy. If you choose trick he eats you. If you choose treat he eats the candy and then he eats you. In the worst case scenario, he eats too much candy and then goes on a sugar-fuelled rampage that can only be stopped by a combined Radiant and Dire force. If you fail, he eats you.
The drama plays out across a new game mode that has Radiant and Dire forces hoarding candy. The candy can be used to distract Roshan until, in the final phase, he goes sugar mad and spawns as a boss that will require cross-team co-operation to defeat.
If you win, Roshan will drop mysterious eggs and essences. Once you've got an egg you can feed it with a combination of essences to change what it hatches into. "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," say Valve on the Dota 2 Diretide page . "Is there method to this madness? Trial and error, and some comparison of results, will surely reveal the rules of Greevil husbandry."
The event brings new costumes, chests, keys and Baby Roshan to the Dota 2 store and Valve have put out a new Diretide trailer to spread the fear.