As it currently exists, it's easy to unlock and cast due to the ability to siphon your own shards, while this is intuitive and good design the problem is that it casts instantly. I think it should work similarly to legendary heroes where it takes a few turns to cast, during which time you should have to defend against an onslaught of armies. This would allow it to be a victory condition without it being an overpowered I win button.
