In the limited time event, it will be heroes (now armed with everything from Captain America's shield to Iron Man's repulsors) against Thanos and a squad of Chitauri aliens clad with laser rifles. The first player to spawn an infinity stone will turn into Thanos, whose special abilities will strengthen as more stones are discovered.
Along with the new game mode, Epic Games, the publisher of Fortnite, revealed a special Black Widow skin for the game, promising another Avenger to get the treatment shortly. Given Marvel will only allow a hero familiar with firearms to be seen with shotguns and sniper rifles, we'd expect it to be Hawkeye, Winter Soldier, Falcon, or Star-Lord.
As always, Fortnite is free to play. So squad up and enter the battlefield.