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. The bats have to be spawned from the Roofed Forest Island.

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