|This page contains spoilers for:||Ultramarines - A Warhammer 40,000 Movie|
Nidon served with the 5th Company of the Imperial Fists, which was responsible for protecting the Chapter Shrine on Mithron and the Liber Mithros contained within, an ancient tome which was purportedly given to the Chapter's care by the Emperor himself at its founding.
The Shrine stood for a millennium, until it came under assault from Black Legion Chaos Space Marines. For three weeks, the 5th Company held the Black Legion at bay, all the while transmitting requests for aid, until a Warp Gate inexplicably appeared above the shrine and unleashed legions of daemons against them. By the time a squad of the Ultramarines' 2nd Company, under the command of Captain Severus arrived to investigate, only Nidon and Chaplain Carnak remained alive, still guarding the Liber Mithros. While both had kept tome secure, its safe keeping was principally Nidon's responsibility.
Nidon and Carnak fought alongside the surviving members of the Ultramarines contingent, repelling waves of Chaos Space Marines while awaiting their extraction by Thunderhawk. But as marines continued to fall and their position looked likely to be overrun, Captain Severus, who had previously fallen off the mountaintop while grappling with a daemon, arrived and seemingly saved them.
However, the real Captain Severus had been killed and possessed by a daemon. Later, after extraction, when Brother Nidon and Chaplain Carnak tended to their wargear in the Armory of the Ultramarines Strike Cruiser, they were confronted by the possessed Severus and Brother Proteus. Severus demanded to see the Liber Mithros, and Carnak, unwilling to sour relations, relented. The Liber Mithros was then revealed to be empty, its pages entirely blank. Nidon stated that he had been aware of this fact and it had been deeply troubling him.
Severus called Carnak and Nidon heretics, implying that the blank text was some trickery of Chaos which they had perpetrated, and leveled a bolt pistol at them. At this moment, the 2nd Company's Standard became alight with corposant fire, and Severus declared it proof that the daemon was present in the form of a possessed Chaplain Carnak. Simultaneously, the Imperial Fists Chaplain suddenly realized the truth - that it was Severus who was possessed - and began to reach for his Crozius, but was stopped by Proteus and shot in the head by Severus. As the dead Chaplain toppled to the ground, the daemon possessing Severus revealed itself. Brother Nidon was first to attack but was thrown aside and knocked unconscious. Subsequently, daemon-Severus fought through the Ultramarines, slaying many.
Shortly thereafter, Brother Nidon regained consciousness and joined his Ultramarine counterparts in hunting the Daemon. Nidon engaged the daemon upon its discovery but was unsuccessful. He was once again knocked away by the daemon, though it is not entirely clear if the blow was fatal or merely rendered him unconscious as before.