Era Telris

Albus Dumbledore is sent a letter and asked to meet a stranger in Hogsmeade, who calls himself Lord Dragonvieux - a title that is completely alien to the Headmaster's knowledge. A proposition is made: "I’ll give you the fall of Voldemort for one Severus Snape and an education." Who is this person? How is he powerful enough to guarantee enough power to stop the terror that a war is based on? What does he want with Severus Snape?