Ai Hino visits her friend Manami Mizushima at Memorial Academy with aspirations of getting enrolled there. When they both head to New London for a night out, the city is under attacked by undead creatures created Necromancy magic, and Ai discovers Manami is actually an Avatar, a group of magical heroes who fights the forces of evil. Ai then meets Tori, a small bird who offers her an invitation to become an Avatar, and Ai learns what the world of an Avatar deals with.
There is a follow up poem call Kame's Song, released exclusive in Arcana Magi Universe Shorts Collection, which is set after the events of Cross.
Arcana Magi Cross is available via this link.
Kame's Song is available via this link
"Here there be spoilers!"