Lantern

"It'll light up your day! Or night. Doesn't matter, you're underground" -Geo

The Lantern is obtained by finding it, but it's not in any dungeons or the shop. Where it resides is up to you to find out, or... you can just look at the Guide section, if you really don't care enough to search... Anyways, its only use is to light up the Black Dungeon, as going in there without it proves a tad bit difficult as you literally cannot see a thing. It is one of the only items that can be placed in the offhand slot and not be cleared. After you find the Lantern, you will always get it back when you enter the Lobby again if you dropped it.