Author's notes: Um. So this occurred to me while I watched the movie. Implied Charles/Erik. The title is based on Nordic mythos. Mimir's Well is the "Well of Wisdom" that will grant you wisdom at a terrible price.
Warning: Contains Spoilers. Obvious character death is obvious.
Edited: I fixed the plot inconsistency and added a bit more. Sorry about that guys. lulz
It wasn't the pain that caused him to continue screaming.
Surely, it started that way. The coin first pierced through the delicate, pain-sensitive skin of the forehead- that was indescribably painful-but when it pushed farther past the thick bone... well, brain matter can't feel pain.
Shaw's fear of death overwhelmed all other thoughts he had. To realize he hadn't become a god, an invincible being of destruction, but remained mortal...
As the coin severed bundles of gray and white brain tissue, the being that was Shaw became no more. He was empty, all that remained was a primal instinct to survive, to kill, to eat. There remained no quantifiably-human, self-aware thoughts as the coin continued its course, through vein and artery alike.
No, it was in knowing that he had lost Erik to the very thing he was trying to prevent that caused him agony. The sadistic edges that Charles had only begun to wear down with peace had for all intents and purposes sharpened, renewed themselves in vengeance. Charles had seen it in the moment before the helmet of silence descended. Erik agreed with Shaw and made an evil promise to their less-evolved brethren. Charles had not been able to feel Erik, but through Shaw he had heard it, had seen it.
The agony of his failure was made worse because Erik's face was calm despite the heat behind his eyes as he used his power to kill. That helmet blocked Charles from knowing, from seeing anything, from stopping Erik before he completely hardened his heart. All the while he knew that without a doubt, Erik was aware that the telepath was lurking behind Shaw's eyes, holding the nightmare in stasis.
And he held on, oh god he held on even as his stomach churned with disgust at the act they committed together, until the tiny bit of metal pushed through the cerebellum. Then, all motor control failed, and Charles Francis Xavier had nothing more to hold on to.
He cried, dropping his hand. The shock of what he had done, just then overtaking him. He was a man more than capable of murder, an accomplice in revenge that had utterly fixed Erik's resolve towards humanity, for the worse. He couldn't damn well see it either!
Just as Charles had been sanding down the edges of Erik's pain, it seemed that in their friendship Erik had worn down his innocence of the world and complicated it with grays far more subtle than he'd ever dreamed. There was a certain madness that love brought in a relationship.
And in love, Charles felt he had not saved Erik at all. In holding Shaw, in protecting Erik, he helped destroy his precious friend in the process.
Was their any hope left for his friend? Was he truly lost in darkness? Charles stumbled out of the plane onto the bright sunlit beach.
He had to believe there was.
End.