Let me explain one thing about hammertime mutator.
Mutators in HOTS are familiar thing to mutators you see in CO-OP SC2, which add new things to missions like.
In Tarsonis train mission, on first mutator was infected marines spawned and acted like zombies. The more you killed enemy units, the more zombies spawned. Also you got reduced sight on mini map and generally on map. Sometimes there is per weekly mutator two challenges, but mostly they are three.
I told you guys this, because that hammertime looks fun.