As the title says, victory by ascension seems to require nothing but 450ish mana or so, a small sum compared to the amounts involved in surviving and conducting drawn out war against the Sorcerer King. It's not hard to accumulate.
Because the spell can be cast repeatedly even in a single turn there isn't even any lengthy period of having to survive intense attack by every faction in order to balance that out.
Any given victory condition should be roughly as difficult to achieve as any other. Ascension victory should require much more mana and in some way take a long time to cast and require the possession of a large number of difficult to defend shards. Perhaps the spell should have to be cast on a majority of the shards while they're in your territory, and all the channeled shards need to be held and defended until the castings are complete with a maximum of one cast per turn allowed, or perhaps even one every other or every third.
Was this not playtested at all? This issue should have been really obvious.