Summary: "Never," said Harry, "The night that your brother died, he drank a potion that drove him out of his mind. He started screaming, pleading with with someone who wasn't there. 'Don't hurt them, please... hurt me instead.'"
--Harry Potter and the Deathly Hallows, JK Rowling
This gives a snippet of a younger Dumbledore's life, as he relived it when he drank the potion.