Hmm. The viral crystals coming from a meteor sort of reminds me of the Command and Conquer series, so perhaps the Eldritch Abomination is similar to the Scrin. Specifically, perhaps the spreading crystalization is the horror's nourishment. The crystals are like the weeds of its world, spreading uncontrollably and choking the world if left unchecked. When the planet is fully enveloped, it fully awakens and begins its feast, the crystals so vast by now that the planet is literally falling apart at the seams. The beast then dips back into hibernation upon a crumbling shard of the planet and patiently waits for it to land upon another planet to sow crystals on. The mental symptoms of infection could be the monster extending its influence over the infectee, perhaps telepathically keeping tabs on the extension of the crystal's spread. The strength of the monster's hold is weak at first, due to the minimal crystals there to resonate with it. But the more the crystals spread, the stronger the monster's grip is, eventually becoming too powerful for the host to throw off. The granted power from those that survive and shake free is actually the fragment of power left behind by the monster, sort of like a beesting, just because the bee is no longer latched onto you doesn't mean the stinger is no longer lodged in your body. The former host's body as adapted and accepted it as a new aspect, and the crystals can no longer harm the former host because they mistake it as the beast.