Nicholas Flamel (born 1340) could be almost as good a source of ancient spells lost to the Interdict of Merlin as Slytherin's Monster

From Chapter 77:

A single glance would tell any competent wizard that the Headmaster has laced that corridor with a ridiculous quantity of wards and webs, triggers and tripsigns. And more: there are Charms laid there of ancient power, magical constructs of which I have heard not even rumors, techniques that must have been disgorged from the hoarded lore of Flamel himself.

So Dumbledore's already using some of Flamel's knowledge in his efforts against Voldemort.