Decayed Bat

Decayed Bat is a item. 5 of them can be given to the Tyashoi Alchemist in exchange for the Great Spook Chestplate.

Obtaining
It is obtained by killing Bats on the Private Island with a Wither Bow and Icy Arrows and will only drop if you have been given the quest from the Tyashoi Alchemist. One of the most simple ways to consistently obtain this item is through the killing of bats in Dungeons. A lower Dungeon floor that is not too small is recommended when getting this item from Dungeons.

Trivia
It's recommended to use the Runaan's Bow to aim and quickly swap to your Wither Bow.

Since Bats in the dungeons are killable with a simple bow shot using the Wither Bow, utilizing an Arrow Swapper to switch between Flint Arrows and Icy Arrows is an easy way to conserve your Icy Arrows when clearing the dungeon with the help of a Bow.