This movie is based entirely on in-game lore, recounted by a blood elf named Melvenor. This movie is absolutely gorgeous and incredibly compelling. I'm very familiar with the story of the Sister Blade, but Melvenor kept the story fresh and inviting. Even using a few files directly from the quest line, The Legend of Sister Blade felt entirely original. Melvenor did a great job narrating his original script, and conveyed the emotion and events without sounding melodramatic or overwrought. He was also truthful and loyal to WoW's lore, which means this is a great video for folks looking for exposure to the backstory of the game.
I am continuously impressed by Melvenor. As a small note, you might recognize the song -- it's Katethegreat's rendition of the Lament of the Highborne. That makes me love Melvenor's work even more, since he's using fan art to create new fan art.