Ascalon is the sword of the original King Thordan. In it is Nidhogg's other eye (not the one the Azure Dragoon has). Even since the original King Thordan died, the eye has been soaking up all the aether the Ishgaurdian's worship has been producing. It's essentially acting like an aether battery for the King Thordan Primal the way crystals do for the other primals we've seen. In the Azys Lla cutscenes, we see Bishop Thordan take the sword out of King Thordan's coffin and absorb all that aether to turn himself and the Heavensward into the King Thordan Primal. And then he wants to absorb all the aether of the Ascians and the Warring Triad...

It's never very clear what is going on with the King Thordan Primal and the KotR. Even with the lore book, there's a lot of contradictions as to what came first and if the Heaven s ward are tempered or not. Given that Bishop Thordan has always had access to Ascalon and therefore Nighogg's eye, I would say that he certainly has enough worship generated aether to mess with the Heavensward. Fully transforming into the King Thordan Primal would give the game away though, and he's smart enough to wait until he's got both the Ascians and Warring Triad on hand.