This is implied in the sentence before - I’m not sure it’s necessary here as well.
as its form wasn’t fully here
would have been able to end this fight long ago
I think this can be made more concise to maximise on the reveal. I’d jump straight from the moment it doesn’t die from the blast of magic into the realisation of who/what this actually is.
When the beast first attacked Cyael had thought it was some ancient minion secretly awakening the Emberheart for its master. Now it was apparent the truth was much worse. He now realized that even though he had held off its attack, he was lucky to still be alive for the beast that rose up before him now was Umvorithiss, the ancient master of the Emberheart itself.
