Solun Decius - Lord of the Flies: In the meantime, the mortally wounded Decius finally lost his fight to the supernatural disease later known as Nurgle's Rot that ran rampant throughout his rotting, corpulent form. Giving himself over fully to the offer of Nurgle to end his pain, Decius' body was possessed by and metamorphosed into the mutated form of the Daemonic entity known as the "Lord of the Flies."His twisted body took on a Daemonic appearance: it possessed an arachnid-like head with numerous eyes and mandibles in place of his mouth, while his decapitated arm grew grotesquely into a great skeletal hand similar to a Power Fist.The Lord of the Flies proceeded to wreak havoc inside the fortress, killing two of his former comrades before Captain Garro strode forth and slew the monstrosity outside the fortress on Luna's barren, airless surface, casting the Lord of the Flies' Daemonic spirit back into the Warp from whence it came.Almost nine standard years later, in the final days of the Horus Heresy, just before the start of the Siege of Terra, the Lord of the Flies assaulted the mobile industrial platform known as the Walking City on Terra. The Daemon appeared as a massive horde of plague flies that infected the people of the Walking City and transformed them into fly-infested zombies. The Knights-Errant were sent to defeat the creature, one of many such Chaos assaults which occurred in the days just before the start of the Siege of Terra.During the battle, the Lord of the Flies used the voice of Solun Decius to goad the leader of the Knights-Errant, Nathaniel Garro, promising him that all his efforts to stop Chaos would be in vain and offering him the chance to rejoin his fellow Death Guard in service to Nurgle. Decius' voice admitted that he had been consumed by a Daemon known as the Lord of the Flies and that was really all that was left of him. The Knights-Errant succeeded in cleansing the Chaos taint from the Walking City, though the Lord of the Flies promised he would yet be the death of Garro.Later, a force of Chaos Cultists struck at the White Mountain, a secret subterranean installation on Terra that was heavily protected against psychic intrusion. The facility was being used to research what had happened to a group of Sisters of Silence who had been captured by the forces of Chaos during the Heresy and then recovered by the Loyalists. In the course of the assault, the Knights-Errant under Garro's command once more moved to defend the facility, and were attacked by the Lord of the Flies.During the combat, the Daemon revealed that after it had been defeated by Garro upon Luna, it had next possessed the body of Meric Voyen, the Apothecary of The Seventy who had once tried to save Solun Decius upon the Eisenstein after his infection. Voyen had been among the 70 Death Guard Astartes who remained loyal after the Eisenstein arrived at Terra. The Daemon laughed as Garro realised that he was once again forced to kill the body of a man who had once been a friend.Garro destroyed the Voyen host body, but the Daemon then moved to possess the Knight-Errant and former World Eater Macer Varren, who had been mortally wounded during the battle for the White Mountain. Varren assaulted his comrades, but froze when his fellow Knight-Errant Garviel Loken confronted him and tried to remind him of their friendship. Varren's soul regained enough control over his own body to grab a belt of grenades from Loken's armour and set them off. Varren chose to destroy himself rather than let the Daemon continue to control him.Free of mortal flesh once more, the Daemon began to hunt for another host, but Malcador the Sigillite finally entered the battle upon the White Mountain's slopes and unleashed his full psychic might against the creature, blasting it to ash. However, the Sigillite warned the Knights-Errant that like all Daemons, the Lord of the Flies could not be completely destroyed while in realspace, and would reform within the Warp.