MSQ Blue moon (spoilers if your not at 3.5 do not click this)

[Felis]

The sole difference appears to be that humanity has not been tempered by their gods, and do not try to summon them... with the events of the Calamity and two attempts now to summon Rhalgr being the exception.

Except Saint Shiva. It don't have to be a god, only something enough people believe.
Because that is what a Primal is, all the wishes and believes of many that became physically with the help of aether.

If one eye was enough to break through a 'certain shield' and two were enough to transform a man into a Primal (who was powerful enough to warp reality in some way), I think it's safe to assume the eyes provide enough aether that matches an enormous amount of crystals that Beastmen wish that could have. All that's left is the worship of those praying to Rhalgr.

Yes
If I remember it right, begin aether to accumulate in the dragon eyes at the moment of his birth. The older the dragon, the more aether is in the eyes and the more powerfull is the dragon. And Niddhog was very old. The eyes are the source of the dragon's power.