WIP, Story idea for a Fantasy story based around the Legends of Avalon and King Arthur.

I started this idea a while back, posting it on steemit but it ran into a dead end. Over the last few days I've returned to it and the story is forming. This is a rough introduction, not edited so after feedback. the story will be told from 2 prospectives, that of a mage of Avalon and that of Mordred, hero and villain.

Introduction.
The Mage.
Deep in the ancient woodlands there is an archway built of stone, nobody knows who built it. It was already ancient when the Druids of the Celts made their offerings there. It is said by some to be a gateway to another realm, some say to Avalon and the Isles of Glass at the centre of the lake Ynys-witrin.
It is said in the old legends that have been passed down locally that the gateway can only be opened from the other side and when people go missing in the area that they have been chosen by the Lady of the Lake to serve in the realm of the undying.
The legends are, in part true. I should know because I have returned from that land where dragons still fly and magic fills the air. A land where the old ways are kept and where the once and future king sleeps, it is said when he wakes Avalon will return.
I have returned from there to stop this happening for when King Arthur wakes and Avalon returns it will be because mankind in our world has fallen and I have to stop this fall of mankind.

Lord Mordred.

I was born to rule, the first-born son of the rightful king who drew the sword from the stone. My mother was the kings half sister, raised as a priestess of Avalon, they coupled in a sacred rite. I was to unite the worlds of mankind and those beyond the mists and preside over a golden age unending.
My father had united the lands of Briton and gathered mighty knights together around a round table and the seeds of the golden age were planted. Then he fell in love with one who followed the new god of the Nazarene and he gradually turned his back on the old ways.
I didn’t meet my father the king until I was fourteen as I was raised on the Isle of Glass in Avalon, taught the old ways and then sent to my fathers court to be trained in the arts of war. By then the influence of the wife of the king had caused divisions between those around the Round Table. The new age was crumbling and the priests of the new god convinced the king to send his men out on a quest to find the Holy Grail, to end the decline of the kingdom.
The land of Briton was falling and when I was old enough I led a rebellion against the actions of my father, but I was betrayed to my death. Avalon who I only ever wanted to serve betrayed me, siding with my father. It was then in a time between my last breaths that I was offered another deal.