The Dragon Valkyrie

The Dragon Valkyrie is a legend in past down on Berk in School of Dragons. Stoick tells it to the player during a quest in the Call of the Death Song expansion pack. It is connected to the Death Song, apparently as its origins.

Story
"A long time ago when we first settled on Berk, a warrior and a valkyrie fell madly in love. She loved butterflies and would spend long hours in the meadows watching them flutter by."

"When she and the warrior got engaged, he vowed to find the most beautiful butterfly in the world as a symbol of his love. He traveled far and wide to find his elusive prize. He finally found it in the nest of an angry dragon horde."

"Vikings and dragons were bitter enemies at the time, so his band of Vikings struck, They fought valiantly, but he fell to the dragons."

"The valkyrie was heartbroken when the men returned with the butterfly... but without her warrior. She fled to the forest, never to return to the village. They say she could be heard crying in the forest, alone and vulnerable...but it was a ruse to draw dragons in. She would attack them to avenge her warrior's death."

"After many years, the trickster god Loki turned her into a dragon with wings like a butterfly that would destroy dragons to avenge the death of her true love!"

Trivia

 * Heather assigns this quest.
 * According to Astrid, Stoick says that the valkyrie and the warrior reminds him of her and Hiccup.