 Re: How did Dumbldore AND Voldemort know?
Well, no one ever said the magical protection was SOLELY based on Lily's sacrifice. I mean, if she was something of the Marauder's Hermione- it's not much of a stretch to think that she could have evoked some powerful charm or rune or potion, whatever, that would be triggered by the ultimate sacrifice? Some ancient and obscure spell, perhaps, though certainly recognizable to such minds and skills as Dumbledore and Voldemort.
The reason that it is not commonly used- simple- it's not commonly known. And furthermore, only applicable under the most dire of situations- not something most people prepare for constantly.
Another possibility- Lily was the only one who discovered while researching, or in some very old book, or whatever- so,
1)she told Dumbledore
2)Voldemort DIDNT know about it- but he encountered its resistance (it seems unlikely he never even tested the wards on the Dursleys) and thus figured out exactly what had happened.
Also maybe something to do with fact that both are perhaps the most accomplished and powerful Legilimancy users.  |