THE BLESSING OF LIGHT
The Blessing of Light does not seem to manifest until we collect the first
Crystal of Light; it seems to be tied to the glyph.
As Hydaelyn's Champion (and in our own right), our strength draws the Crystals of Light to us, and they in turn further strengthen us. It's a positive feedback loop. Many characters remark that it's amazing that our strength could not only draw one crystal, but that our strength in addition to it would draw another... and another... As far as I can tell, the Blessing of Light is essentially the glyph we see in the visions, strengthened as we collect the crystals. A guess would be that their power enables Hydaelyn to shine through them and shield her champion in the corporeal realm, but doing so has come at a great cost. With Hyadelyn weakened, we still lean on her like a crutch, and I think this offends Midgardsormr, who entered into some covenant with her long ago and sees us as weak and unworthy to be her champion. By sealing the weakened Blessing of Light and binding himself to us, I think Midgardsormr is engaging in a trial of mankind through us, much as Ramuh did, to see if Hydaelyn chose well or if we'll just burn along with Ishgard.