Image imported from derpibooru 
Original Description:
 
White Twilight is seen in several episodes. She is not a true emoticlone, but actually an embodiment of Princess Twilight at full strength and power. She is first seen when Princess Twilight unites with all of her emotions in Nevermore to combat her Rage, and Twilight herself later turned white during her brief friendship with Lord Tirek after the world had come to an end. In this case, this form represents Princess Twilight when she draws on her entire inner strength and power or when she is in emotional harmony with herself, which she has displayed only in relation to her closest friends. It also represent her spirit being cleansed of her centaur evil. In this form, Princess Twilight emits a white glow, and during her battle with Lord Tirek her hair grows to waist-length.
 

 
1st Appearance: In the episode “Nevermore”, Raven unites with her emotions and becomes a gigantic white version of herself to subdue her Rage.  
2nd Appearance: In the episode “The End - Part 3”, a younger version of herself wearing white clothing was present throughout the episode and after her body and memories were restored, she once again became White Twilight to defeat Lord Tirek once and for all.