A Battle Lost to Ashes

It's a secret that does not leave here-- of how the Nightwing Sword, the White Phoenix, and the Song Arcanum saved the Kingdom of Arvanoc. They saved a kingdom and didn't even know it.

— Professor of Darcanis

From the "Journal of Kazi Medoa, the Song Arcanum"

The gate to the hidden cathedral was nothing special- an ancient looking archway that Aen ran past only to step back and re-examine. I have no idea what possessed him to do so, but perhaps it was all the time spent with the Nightwings-- doubting that every shadow existed and everything was at seemed.
On second thought, it was strange that the archway looked cracked and damaged for it was the only thing in this prisitine temple that was. Aen seemed to run things through his head before telling me that it was a mix of Enochian and something that he hadn't seen before but had read about-- ancient script on a spire of time, I believe.
I've never seen a man move so fast before, taking out what looked to be some sort of crystal before carving a rune into the stone. How he figured it out I believe is because he knew the language-- knew what the rune to unlock everything was.
Anyhow, he was right-- that's all that mattered. The archway opened with familiar realmwalking magic revealing an icy bridge up in what he immediately recognized as the Spine.
"We've flown past this on patrols." he told me as I looked past him. "It- It was always empty and frozen."
Before us was a grand cathedral carved into the mountain clearly by something horrible and shaped by someone- or somethings- definitely fiendish in nature as its outsides looked twisted and gnarled, burned and pointed. I've never seen something so ugly as it was but what was obvious was... it wasn't frozen. It looked alive... twisting and writhing in the light of Bryga and Dhirga.
"I think you and the others were tricked, my friend." I hazarded to guess.
He didn't like that and I could understood why, it unnerved me so as we began to run across the bridge that this true facade was never found.

The Lost Song

The bard's account of the battle was lost and torn from the journal, with no spell being able to recreate its pages-- even a Wish spell had been used by a powerful mage at one point to unlock the secrets of what had been faced and it was all for naught.
However, other historians from recent times have compiled somewhat of a timeline and a story from other accounts Medoa went on to sing about believing that there were clues hidden in all his songs and his journals about what truly happened as they say he had a lost song that could very well shape the futre of Arvanoc itself.

From the "Shorthand Notes of Unsolved Legends" by Cinta Valin

A Summary of Events After the Journal Passage...

Strung up and suspended high above the waters was the White Phoenix, Saelyhn, captured and prepared to be drowned in what looked to be a shallow pool of water. However, Medoa had noted that the intent was clear as day, she was to be consumed by whatever would be summoned to lay in its depths.
When the Nightwing Sword and the Song Eternum infiltrated the cathedral, having superceded the abjuration magics on the gateway and thus raising no alarm, they set forth laying traps amongst the ritual spaces, joining in the movements of the congregation as they chanted for something which has not yet been established to this day.
Just as she was to be lowered into the pools, the Nightwing Sword flew in as explosions erupted from the crowd sending cultists flying into pieces. However, the ritual involved much more than what they had expected. With the White Phoenix free and by their side, a battle raged on of fire and blood, three versus the cathedral where even the walls itself seemed to turn on the heroes.
When all was said and done and blood seeped into the shallow pool-- when the heroes thought themselves free of the darkness nearly summoned-- something crawled forth. Abyss-black tentacles wrapped in shadows sprung toward the nearest of the light, wrapping itself around he that the Nightwings held so high dragging him into the depths while the others could not pursue, prevented from continuing as if the bottom existed for them-- yet they could see. Eyes blue and cold and a shape in the distance of the dark pool, that rippled in the water.
The Ender of the Worlds, Medoa called it. Apocalypse incarnate.
The blood had called to him and while not enough to sustain him, he would take what he was owed from our realm in order to become stronger. However, the hero was not without his wits, with what last remained of his connection to his friends, he weaponized a potion, a spell, and with the last of his breath, escaped. We know not how but we do know that the shadow was not pleased.