Surprise got her hooves on an ancient book of mysterious potions. When Pinkie Pie got wind of the adventure, there is no stopping what is to come next, just as there is no telling of what was to come out of it.
The two Ponies managed to gather the ingredients, before they started to perform what the recipe of the book described. Even if some of the details did sound strange to the point of being creepy.
As it turned out, all in good fun. Yet, how far is a Pony prepared to go in order to have just that one more friend?