Planeswalkers of the Multiverse: The Shadow of Eternity
The Eldrazi are a race of transdimensional beings that feed on the life of the planes they travel to.
On Innistrad, the Eldrazi titan Emrakul invaded and wiped out much of the life on the plane. After the Gatewatch defeated them, they disappeared. However, months later, they have returned in force.
New cultist groups have grown around this new Eldrazi insurgence and does not just threaten Innistrad, but the entire multiverse.