what are they. the names are mentioned on the Devil May Cry wiki as weapons used by the Order of the Sword but nothing i have seen explains what they are.
Caliburn = Excalibur: self-explanatory.
Durandal = sword of Hector, from Trojan Wars, containing a thread from the cloak of the Virgin Mary, a tooth of St. Peter, a hair of St. Denys, and a drop of St. Basil's blood. It was unbreakable
The artbook. If I remember correctly, Caliburn is the standard sword, Durandal is the officer's sword.
I have complete scan of Japanese artbook of DMC 4. I don't know if these swords are in it.