Vampire

Vampires are Hardmode enemies that spawn only during a Solar Eclipse, during which they are as common as a Zombie. Vampires will turn into a bat if the player is above them and attempt to fly near the player and turn back into a humanoid.

Tips

 * Due to low lighting during a Solar Eclipse, it may be difficult to see this enemy before they strike you. Vampires in Terraria typically fly in as a bat which is relatively hard to see without artificial lighting. Try using light buffs (Shine Potion, Wisp in a Bottle) and watch for them as they fly in to prevent unnecessary damage.
 * The Stake Launcher is a highly effective weapon against vampires, as it does extreme damage to them.

Trivia

 * Being vulnerable to the Stake Launcher is obvioulsy rooted in Vampire mythology/culture, where stakes are the usual method of destroying vampires.