Loki has just been through his sentence on Asgard and he is in a emotional breakdown. He is restricted from going anywhere, but since the Chitauri are still after him and he owes them quite a deal, he decides to be unfaithful and goes to the Asgardian library to look up a book called "The big book about the Nine Worlds and further". He skims through the book and finds a world called Equestria, he decides to go there and when he arrives, he teams up with Discord to conquer this world, once and for all. (and to make the ponies kneel for him)