"The Five", a group of Vampire Hunters, were created by an unknown dying witch. They are highly skilled and the spell that created them gave them tattoos that cover their arms and chest.
The witch gives them power to fight vampires. These tattoos are supposed to be a map for an ultimate weapon to destroy vampires. This weapon could be a way to make vampires human again (either that or Alexander lied to Rebekah to gain her trust).