Bloodbath

Blood Moons happen randomly at night (about 1-in-9 once anyone has 120+ HP) - the screen goes red and zombies can open doors. Surviving is the whole assignment: wall yourself in, block the door with a table, and let dawn come. Or fight on the doorstep for the unique drops - the event is also the home of Til Death... and, in hardmode, the nasty fishing enemies like Don't Dread on Me.