It's called the SHARK BLADE, a giant metal shark fin pops up and does a full circuit of the room you're in.
I love how this game is structured. Your EXP and trap collection is shared across all modes, so if you're having trouble in the story mode, you can do a bunch of the one-off challenge maps to get extra EXP/traps, and vice versa.
The challenges the witches give you in the story mode are also amazing, they are like HEY WHY DONT YOU TRY BURNING SOMEONE ALIVE and I'm like sure that sounds fucking great