You can now play an additional game mode in Dota 2, it's called Diretide. This fun gamemode has been implemented in the new Dota 2 seasonal patch starting this two-week Halloween seasonal event.

In the second phase of Diretide, the aim of getting more candies than your enemy gets complicated, because an invincible Roshan is hunting single heroes and can only be stopped by feeding him with more candy. Both phase 1 and 2 last for 10 minutes each.
Ultimate goal: Kill 150,000 HP Roshan
After these two phases, one team is declared the winning team, but the game is not over: Roshan will spawn with 150,000 hit points and is the target to kill for both teams together. If you kill him, there are even more Dota 2 items dropping for both teams.
The current record for killing Roshan is at 1 minute and 17 seconds. Can you kill Roshan faster? In addition to this Diretide game mode, the standard map has been Halloween-themed and you can hatch a Halloween egg eventually spawning a courier with custom colors.
Source: Dota 2 Blog