Starting today and running through November 6, Hearthstone is hosting a Halloween-themed Hallow's End event.
Any player that logs into the game between now and October 30 (10 PM PT) will receive a Whispers of the Old Gods card pack and an Arena ticket. If you log in again anytime during October 31 and November 5 (10 PM PT) you will receive one Knights of the Frozen Throne card pack as well as another Arena ticket.
For Hallow's End, Hearthstone will be mixing up the dual class Arena Mode by giving each of the nine classic Hearthstone heroes a costume. During the Hallow's End Arena event, you'll choose a hero and a separate hero power and the cards you can choose from will come from both of those classes. So if you choose a Warlock as a class, but choose a Druid's power, you can choose cards from both classes. That's on top of the neutral cards you can pick from.
Secondly, there's The Headless Horseman Rides Tavern Brawl that will run from October 25th to the 28th. You'll be able to challenge the Horseman to a dual, but only in disguise. That's why after the Brawl starts you'll be given a costume, each with its own special deck and a new passive Hero power.
So log in today if you want to partake in Hearthstone's expansive Hallow's End event.