This gamemode was added as part of The Walking Dead crossover.
Walker Attack! is a Player vs Enemy gamemode, where two players must work together to survive waves of attacking Walkers.
Walkers are zombie-like creatures with low amounts of health that attack you with unarmed attacks. They will follow you until they get close enough to attack. As the waves progress, more Walkers will spawn per wave, and they will become more difficult, performing stronger attacks and even dodging.
This gamemode is a timed match with a set amount of lives. The game is over once the timer runs out or if both players lose all their lives.
While the goal of this gamemode is to get to the highest wave, the winner of the match is sometimes determined by who KO'd the most horde enemies.