What monster killed dumboldre?

Dumbledore was killed by Severus Snape in "Harry Potter and the Half-Blood Prince." Although it may seem like a betrayal, this act was part of a larger plan agreed upon with Dumbledore to protect Draco Malfoy and maintain Voldemort's trust in Snape. The killing took place atop Hogwarts' Astronomy Tower, where Dumbledore was weakened by a curse from a Horcrux.