This takes place not long after Crusaders of the Lost Mark.
Rarity's new love interest is discovered to be sick and psychopathic monster. Her injuries force Princess Twilight to reveal her true feelings to the beautiful Unicorn.
This causes a cascade of affects that may be felt in two worlds. Causing ponies and others to examine what they believe about friendship and love. All the while, Rarity's deepest dreams come true in a most unexpected way.
Meanwhile, the pony who started all this tries to disrupt any happiness Rarity has found. How far will Twilight go to protect the ones she loves?