User blog comment:D. Cello/AC: Revelations Gameplay Trailer/@comment-3967387-20110818011356/@comment-4013990-20110818090450

I just posted this at the top of the blog, but I think it's the time when a younger Altair rescues his Master during a Templar attack. I think they mention it in Assassin's Creed: The Secret Crusade (the book of AC 1).