Hallow's End has made its way into Heroes of the Storm with a handful of new Halloween-themed hero skins: Countess Kerrigan, Skelethur Abathur, and Infested Tychus. Technically Infested Tychus is not a Hallow's End skin, but rather Tychus's Legendary skin, but it's creepy nonetheless. If you aren't in the alpha, no sweat. The skins are a permanent addition to the game and you'll be able to buy/acquire them when the game goes public if you so desire.
Countess Kerrigan is my favorite of the lot because it plays into one of the alternate universes presented by select alternate skins -- Vampire Slayer Valla's description specifically mentions the Countess. You can imagine Vampire Slayer Valla pursuing Countess Kerrigan in a Victorian Van Helsing-y landscape. Considering the characters are from two completely different games, it tickles the imagination. Throw in World of Warcraft's San'layn as a haughty aristocracy and the scenario could support a game unto itself.