Decayed Bat

The Decayed Bat is a item. There is a 100% chance of the Decayed Bat being dropped from a killed Bat if the required conditions are met.

can be given to the Tyashoi Alchemist in exchange for the Great Spook Chestplate.

Obtaining
It is obtained by killing any kind of Bats 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 ways to consistently obtain this item is by spawning bats by using a Roofed Forest Island on your private island, and creating a box surrounding the crystal. Note that it must be night time for bats to spawn on your private island. Another way is to do lower Catacombs floors, and kill the bats found as Dungeon Secrets.

Trivia

 * It's recommended to use the Runaan's Bow to aim and quickly swap to your Wither Bow in order to hit the bats.
 * There are a couple requirements in order to obtain the Decayed Bat from bats:
 * The player must have talked to the Tyashoi Alchemist to receive the quest.
 * The player must have Icy Arrows in their quiver, and have selected them with the Arrow Swapper.
 * The player must shoot a Bat with a Wither Bow.