>Codex: Heretic Astartes page 47
Of the fate of the Primarch Fulgrim, none are sure. The enemies of Slaanesh claim he was killed during the battles against his fellow Legions, but robot-crewed Mechanicus trawlers recovered neither his body nor the remains of his Battle Barge. Among the remains of the Emperor's Children, it is rumoured that he was rewarded for his devotion to pleasure and elevated by Slaanesh to become a Daemon Primarch, lord of a Daemon world.
>Also Codex: Heretic Astartes page 14
Fulgrim, the Daemon Primarch of the Emperor's Children, leads his hedonistic armies in a spiteful strike against the Ultramarines upon Thessala. Amongst the swirling pink mists of Slaanesh's favour, Fulgrim hunts down his brother Primarch Roboute Guilliman and fights him in single combat. Though Guilliman is mighty indeed, his opponent is a serpentine giant swollen with daemonic power, each of his four arms wielding a poisoned blade.
>The Gathering Storm Part III, Rise of the Primarch page 43
As Guilliman stared in disgust at this cursed fetish, he heard a susurration hissing within his mind that he had not heard since that fateful encounter on Thessala. In mocking tones, Fulgrim welcomed Guilliman back to his beloved Imperium. The Daemon Primarch revealed that he had concealed a fragment of his own animus in the amulet that his servant wore, and confessed disappointment that Guilliman had rejected his gift, the Crown of Glories. Many heroes great and pure had fallen to the trinket’s blandishments, and Fulgrim had hoped that he could corrupt Guilliman in the same fashion. Yet the Slaaneshi Prince assured his brother that this was but the first of endless temptations that Guilliman would have to face. Laughing cruelly, he taunted that the Lord of Ultramar would never be able to trust any feeling of triumph or self-satisfaction again.
Of the fate of the Primarch Fulgrim, none are sure.
Seems pretty clear to me
>three different entries from three different time periods by three different imperial scribes
>different
wew
>>55034242
>le everything is written in-universe meme
>>55034242
They haven't had codexes be written from an in-universe point of view in several editions.