The Horde Bash event went live 10/5/17, delayed a few hours due to a storm shield vault wipe. The gameplay is similar to a storm shield defense, with more waves.
Types of waves
Normal waves consist of Husks, Husky Husks, Pitchers, Beehives, and Lobbers, with the occasional Mist Monster and Shielder.
Lobber waves have more lobbers than usual, destroying weak bases.
Smasher waves contain many more smashers than normal, and multiple can spawn at the same time.