Vespasian was one of eleven Lord Commanders of the Emperor's Children Legion during the Great Crusade and the beginning of the Horus Heresy. Unlike his counterpart Eidolon, Vespasian was immensely likeable, for his incredible abilities as a warrior and commander were tempered by a rare humility for a Astartes of the III Legion that made others warm to him immediately. Following the Cleansing of Laeran and the onset of Slaaneshi corruption within Fulgrim and the Emperor's Children, Vespasian took note of the change in character of the Legion as a whole. Over time a rift grew between the once-favoured Lord Commander and his Primarch. When Vespasian attempted to seek audience with his lord, his efforts were repeatedly rebuffed. When he eventually confronted Primarch Fulgrim over the rot and corruption that plagued the Emperor's Children, he finally realised the full extent of the malign power of Chaos. Fulgrim murdered the Lord Commander with the poisonous Chaotic blade known as the Kinebrach Anathame.
Following the Cleansing of Laeran, the rot of Chaotic corruption had begun to spread throughout the entirety of the III Legion. Lord Commander Vespasian, one of two officers with the rank of Lord Commander within the rigid hierarchy of the Emperor's Children, staunchly adhered to his Legion's ethos of perfection through honour, refusing to allow himself to fall to the rampant hubris and arrogance that had become more common within the ranks of the III Legion as the Great Crusade progressed. Over the course of many months, Vespasian consulted other like-minded Battle-Brothers within the Legion, including the 2nd Company Captain Solomon Demeter, about his growing concerns and fears for the future of the Legion.
Vespasian was present during the meeting between Ulthwé Craftworld Farseer Eldrad Ulthran and his Primarch Fulgrim upon the Eldar Maiden World of Tarsus. He had heard the the dire warning presented to them of the Warmaster Horus' corruption by the then-unknown Ruinous Powers of Chaos and the inevitable rebellion and civil war that would follow. The Primarch had rejected such accusations outright. Influenced by his Laer daemonblade, Fulgrim ordered his delegation to attack the seditious xenos. Lord Commander Vespasian reluctantly followed his Primarch's orders and destroyed Tarsus and the rest of the Eldar Maiden Worlds the 28th Expeditionary Fleet had recently encountered using Exterminatus. Weeks later, Fulgrim's fleet was ordered by the Council of Terra to rendezvous with Horus' 63rd Expeditionary Fleet to inquire about reports of the Luna Wolves' recent conduct and acquire information in regards to the grave injuries Horus had sustained on Davin's feral moon. Vespasian was excluded from this delegation to the Warmaster, further demonstrating the growing rift between the Lord Commander and his Primarch Fulgrim.
Following this visit to the 63rd Expeditionary Fleet, Fulgrim announced to his senior commanders that he would lead a small force to join Ferrus Manus and his X Legion at Callinedes IV in the Callinedes System under the pretense of clearing it of an Ork infestation. The rest of the Legion would rendezvous with the Warmaster’s 63rd Expeditionary Fleet at the Istvaan System to halt a growing rebellion. But the III Legion wasn't to fight alone, for they were to deploy alongside the Death Guard and the World Eaters. Though the men of his Legion cheered in adulation at the prospect of fighting with their brother Astartes, Fulgrim's amusement turned instantly to sorrow as he understood that, but for Vespasian’s stubbornness, a great many of these warriors would have made a fine addition to the army of Horus' planned crusade against the Emperor. Vespasian’s refusal to allow his men to sample the heady delights of Chief Apothecary Fabius’s chemical stimulants and surgical alterations, had condemned the warriors under his command to death in Horus' trap on Istvaan III since they could not be trusted to betray the Emperor. Fulgrim realised he should have disposed of Vespasian much sooner, and the mixture of guilt and excitement at the deaths he had set in motion proved to be a potent cocktail of sensations. When the planned massacre on Istvaan III was done, the Loyalist chaff would have been cut from Horus' forces, and they would be a sharpened blade aimed at the heart of the corrupt Imperium.
Vespasian met his end after the operation conducted by the III Legion to cleanse Callinedes. The Lord Commander was denied audience with the Primarch, a most unusual occurrence, after numerous requests. A dark shroud had hung over Vespasian ever since the battle on the deep orbital of the Callinedes System. The Primarch spearheaded the assault during the pacification of the Diasporex's Deep Orbital DS191, leading the 1st, 2nd and 3rd Companies. During the assault, Captain Solomon Demeter, commander of the Emperor's Children's 2nd Company, found himself unsupported by the 1st and 3rd Companies as the battle-plan had required. Overextended and in real danger of being cut off and destroyed, the 2nd Company was only saved from certain destruction by the unplanned and timely arrival of the 10th and 13th Companies under the commands of Captains Saul Tarvitz and Lucius, two junior officers that Demeter had found himself associating more and more with as he was systematically frozen out by the III Legion's high command. A dark shroud had hung over Vespasian since witnessing these events and he felt obliged to act after he observed Solomon Demeter's 2nd Company being intentionally abandoned by both Captain Kaesoron's and Vairosean's companies.
Within hours, the 2nd Company was in transit to the Istvaan System to rendezvous with the three other Legions to pacify the rebellious world of Istvaan III. Finally seizing the opportune moment, Vespasian confronted Fulgrim in his stateroom on his flagship, the Pride of the Emperor, over the growing corruption and decay of the III Legion's ideals. The Lord Commander discovered, much too late, the malign influence of Chaos that had held sway over his Primarch. With the sibilant whispers of the daemon-possessed Laer sword urging him on, Fulgrim murdered Vespasian in cold blood by stabbing him in the neck with the Kinebrach Anathame that had been gifted to him by Horus.
- Fulgrim by Graham McNeill