The Vampire's Catalyst is a Transformation tool that is obtained from the Treasure Bag of the Viscount in Expert Mode only. When used, it transforms the user into a Bat. While transformed, the player can fit through 2-block tall gaps and has a top speed of 26 miles per hour, can fly up to 183 blocks high, but cannot use items with the exception of Soul Link. In addition, while transformed, the player will not take any damage from bat enemies.
Like other Transformation items, it can go into the Mount equipment slot, and as such cannot be used while the player is on a Mount.