When the tired caravan finally reached the old castle of Canterlot, they were quickly helped in under the protective spell raised by Shining Armor. Now they had time to plan for a way to deal with the changelings, and what other horrors that awaited them.
Unseen Invasion is the first part of a 3 part MTG expansion set, intended to tell an overall storyline through ponified Magic the Gathering cards, exploring the plane of Equestria and its struggle against a darker force invading it. Unseen Invasion is a quick prologue-style set, with the main story beginning in the second set, named Finest Hour.
Unseen Invasion begins here Read it if you want the back-story of the main set.
From what I understand, being able to resolve a 8 mana mythic rare spell should spell instant win anyways, but I get your point, it is rather strong. A way to solve that could be to make it two-sided (no players can take damage). Or do you have a suggestion of your own?
Making it twosided would make for quite a buildup of forces, which does seem like a rather cool idea. But if you think it's fine like it is, don't change it