Nemsy Necrofizzle is a new alternate Hearthstone Warlock Hero that can be claimed for free by players taking part in a Fireside Brawl during a Gathering. The event to claim Nemsy begins on October 17th. Fireside Gatherings are public events during which players can "Brawl" in a Tag Team event (mini 3v3). Anyone can host a Hallow's End Fireside Gathering by visiting the Hearthstone site and setting one up on the specially dedicated page.
Events will need to take place between now and November 5th and "kits" can be picked up to help organizers select Innkeepers and so forth.
This Hallow’s End, you should also expect a magical guest. Hearthstone’s newest hero, Nemsy Necrofizzle, will be available for free at established Fireside Gathering Taverns starting on October 17*. This pint-sized Warlock will creep her way into your Hearthstone collection if you play any Fireside Brawl at a Fireside Gathering Tavern. Even though it’s Hallow’s End, have no fear—Nemsy will continue to be available at Taverns even after this spooky celebration meets its . . . end.
You can learn more on the Hearthstone site.