Reaper Hood

Goodie Bags are the only source, and those only drop while Halloween is on. The route to the outfit is ordinary night farming during the event with everything opened as you collect it, not a hunt for one specific monster. Nothing else in the world produces it, so a missed October costs you a year.

It is the folklore Grim Reaper: skull under a black hood, long robe, scythe. The scythe is part of the costume rather than a weapon you can swing, and it stays on screen permanently, including while you are using an item. That is the one thing to know before equipping it, because the blade will sit over the animation of whatever you are actually firing.

On old-gen console and 3DS both character sprites use the same art, so the outfit looks the same no matter which body type you picked.