In truth, the real Ossuros died early on in the Great Crusade, but his identity and memories were taken by his successors though the abilities granted by their Legion's overactive Omophagea gene-seed implant. They allowed his legend to live on through them until their own deaths.
The last incarnation of Ishidur Ossuros fell during the first battle of the newly-named Blood Angels with their Primarch, when he gave his life to save Sanguinius from being overrun and slain. Ossuros' body could not be found, so his persona could not be passed on to a new Astartes through the consumption of his flesh.
The first and last master of the IX Legion, other than Sanguinius himself, was a warrior who fought under the name of Ishidur Ossuros. His is a legend that spans nearly 80 standard years of war, fighting in battles from the rad-wastes of Old Earth to the rocky heights of the world of Saiph, a legend of blood and death that charts a course across the most hazardous battlefields of the Great Crusade.
He was the hero of the first assault on Kum-karta in Yndonesia during the Unification Wars, where he reaped such a toll of lives that his warriors emerged from the fortresses' sunken vaults painted in crimson from head to foot, and the champion of the Ghost Wars on Saiph.
If the records of the Imperium's original Divisio Militaris are to be believed, Ishidur next fought in 317 separate major engagements -- and yet these same records also show his death on at least four verified occasions.
Given the grim nature of the early IX Legion in the period before it was united with Sanguinius, and those few oral records from the warriors of the Legion itself that still exist, it is likely that the true and original Ishidur Ossuros fell in battle as early as 802.M30.
His lieutenants then took his memories and his name through the use of their mutated Omophagea implants, and continued his legacy for the sake of the Legion. Each time Ishidur Ossuros fell, he rose again like a gore-soaked phoenix, once more to take the reins of the IX Legion.
The last known reference to this warrior is during the Battle for Teghar Pentaurus, the first battle fought by the renamed Blood Angels under the command of their Primarch. There, the enemy's last gasp was a ferocious assault that threatened to overrun the Primarch and his Honour Guard. It was the final Ishidur and his warriors who would save the Great Angel, staging a suicidal counterassault into the teeth of the foe and holding them at bay for almost a solar hour as reinforcements rushed to Sanguinius' aid.
Ishidur Ossuros perished one final time, his body never recovered, exchanging his long life for that of the Primarch in whom he had entrusted the future of the IX Legion.
- The Horus Heresy - Book Eight: Malevolence (Forge World Series) by Neil Wylie and Anuj Malhotra, pg. 123