The Princess Ending is an Ending in Prisoners of the Elder One. It occurs after examining the Mirror of Altenak four times while being anywhere but the Nexus.
The more Erik looks into the mirror, the more his reflection seems 'off'. By the fourth time he looks in it, he is transformed into a Princess of another world, reality shifting to match. In time, a knight rescues her and they live happily ever after, but Tiffany isn't so lucky.