Istvaan III... it was here, where the Imperium of Man's greatest challenge began. When Horus Lupercal revealed his treason to the Imperium when he ordered those within the Traitor Legions who maintained loyalty to the Emperor purged.
This planet would eventually be forgotten by the greater Imperium. Overshadowed by the fires of the Horus Heresy. Only remembered by the Space Marines... and their fallen kin.
In time, an expedition of the Thousand Sons rediscovered the planet, and to their surprise... found a survivor of the ancient holocaust.
Rylanor, the Ancient of Rites for the Emperor's Children, a powerful Dreadnaught of the Great Crusade. He had laid in silent slumber since the fall of Istvaan III until the Thousand Sons found him. This brought the attention of the Traitor Primarch of the Emperor's Children, Fulgrim.
Rylanor revealed that he had a virus bomb in his possession. The two former friends battled, as the Thousand Sons present used their considerable powers to stop the ancient doomsday device from igniting.
So great was Rylanor's valor. That even the long damned sons of Magnus the Red were moved and defied Fulgrim. Permitting the bomb to detonate, again destroying all life on Istvaan III, and wounding the vain Primarch's soul.
But this was not the true end of Rylanor. For in secret, the Penitent Scions. Descendants of the ancient Thousand Sons Legion that stood beside the Emperor had tailed their kin. And through their own psyker might. Observed their former brothers from afar. And, right as the virus bomb ate away at the life on Istvaan III. They opened a portal for Rylanor, spiriting away the Ancient of Rites.
"What... what has happened?" Rylanor asked, bewildered at suddenly being within a bay of a battle barge, rather than facing his gene-sire as death overcame Istvaan III a second time.
Then entered a Space Marine, a large man even for the Adeptus Astartes, and what three eyes. "I apologize, oh Ancient of Rites."
"What is the meaning of this?" Rylanor demanded.
"You are owed an explanation. My name, Tria Matia, Chapter Master of the Penitent Scions." Tria introduced. "We followed a Thousand Sons ship outside of the Eye of Terror to this world... and to our surprise. We found ourselves in the infamous Istvaan system... and through our 'gifts'. We learned that a champion of the Great Crusade had survived... one that our brothers in the Ash Lords would not forgive us for allowing to die."
"That is not you're choice to make!" The ancient Contemptor Dreadnaught bellowed.
"I suppose you'd say as much to Fulgrim. His presence, even as far away as we were was like staring into the heart of a sun, and the whispers... my point is he survived your virus bomb." Tria explained.
"Then... my vengeance is not yet complete." Rylanor seethed with rage.
"True, but you need not pursue retribution alone," Tria said. "For there are still those of the Phoenix's genes that stand against their fallen Primarch. Chapters of the Space Marines that have survived, with the Phoenicians gene-seed.
"What are you even talking about boy?!" Rylanor demanded, unfamiliar with what 'chapters' were in this context.
"I see... I suppose it would make sense that you would not know." Tria said in realization.
In time, the battle barge returned to the Fortress Monastery of the Ash Lords. A volcanic world. Known as Ash Haunt.
Tria and a contingent of Penitent Scions landed at the Fortress Monastery of the Ash Lords via Thunderhawk. Tria was greeted by his counterpart among the Ash Lords, Chapter Master Reverens. "Lord Tria, what brings you to our home?" The robe-clad Space Marine asked.
"I come... to offer something, that I have no doubt will interest you," Tria said, then Rylanor disembarked the Thunderhawk.
The Ash Lords were surprised. "What... this... this is an Emperor's Children Contempter Pattern Dreadnaught!" Reverens said.
"Yes... untainted by the Ruinous Powers, and. What's more... recovered from the Istvaan System." Tria said.
The Ash Lords looked at the Chapter Master of the Penitent Scions with disbelief. "Tria has told me much about your Chapter. I for one am proud that, at the very least a few of the Emperor's Children escaped the perditions of Chaos. I am Rylanor, the-"
"You... you are a miracle of the Emperor." Revenens and the rest of the Ash Lords fell on their knees in reverence for this ancient and almost forgotten hero of the Imperium of old. "Tria, no act we can commit will ever fully convey our thanks for this honor."
"We are brothers, though our gene-fathers have fallen into blackest damnation. That is no reason to not give gifts freely." Tria said. "And for a warrior of Rylanor's Calibur... someone who faced the traitor Fulgrim himself in combat. I can think of no better gift for you and you're warriors, Lord Revenens."
"I... am not... a possession," Rylanor said with scorn.
"Apologies, Ancient of Rites," Revenens said as he and his brother stood up. "It is just... such an honor that you are here... and... and I am sorry. I have always strived to follow you're example.
Rylanor's mood turned to one of disgust, to respect. "I see... where am I needed most?"
Several days later. Rylanor was in an Ash Lords drop pod, that had just landed on a planet under attack by the Tyranids. His armor was repainted in the grey colors of the Ash Lords. The only remnant of his former life. Being a single pauldron with the Pre-Heresy colors of the Emperor's Children, and the Aquila symbols that his former Legion were permitted to bear by the blessing of the Emperor. "These foes are unknown to me, but they shall perish as any other enemy the Imperium has encountered."
"Till All is Ash!" The Ash Lords cried out as they engaged the monstrous aliens.
"Rejoice! For the Ancient of Rites fights for Humanity once more! REMEMBER ISTVAAN!" Rylanor charged into the swarm with reckless abandon, slaughtering the monstrous xeno's with the same fury and vigor he fought with, in the Great Crusade of yore, as did the battle-brothers of the Ash Lords.
Over time, stories would spread across the Imperium of a ghost of the Great Crusader, a loyalist of the Traitor Legions fighting beside his gene-kin. Who faced his fallen Gene Sire, and lived to tell of it. A Champion of the Emperor, inspiring every soul that stands beside him, and a terror to those who stand against the Imperium.
He is Rylanor, once of the Emperor's Children, Ancient of Rites, Venerable of the Palatine Host, proud servant of the Emperor, Scourge of Fulgrim.
Authors Note: I have no idea if it's feasible for Rylanor to be spirited from his death without Fulgrim noticing anything off. But still, a veteran of the Great Crusade being saved is a good concept for a story.