Epilogue Two: The Rangda Xenocide
Dante Uriael and the Dawn Angels are cornered and alone deep in Rangda Space. The Rangda accomplish this by using something called the Black Miasma. Which is a moon-sized gelatinous mass of Pariah tissue produced through industrial "farming" of Pariah beings. Turning them into massively balls of still living Cancer that grows constantly. The Black Miasma messes up Psychic Foresight and dampens psychic powers in a light-year-sized area.
Dante is badly wounded and has one of his wings ripped off, and the other burnt. The legions are suffering massive casualties and are only saved by the Wild Hunt's arrival. Tyric brings with him some prototypes from the Ordo Sinister and them working with Dante manage to punch through the Black Miasma and destroy it. Tyric and Dante face-off with Opus-Jorith. Highlights include Tyric conjuring up a blizzard in the void, and unleashing giant spirit wolves made of liquid hydrogen and other gases against Rangda Macrobeests. Dante almost gets killed but is saved by the Sanguinor, giving him a psychic peptalk. He regrows his wings and is briefly possessed by Sanguinius who manages to kill Opus-Jorith. Sanguinius leaves Dante with some cryptic hints about the first Doom Tide before fading into his soul.
The Dawn Angels and Wild Hunt meet up with Eddard and Imperial command. After about a decade of very very ugly warfare with the Imperium conducting literally hundreds of Genocides on Rangda infected worlds. Alpharius Omegon and his Legion work with Moric Thane and his Legion to steal data on Rangda Prime.
Rangda Prime, the system is deep in the Halo Stars and is a collection of H.R. Giger-style megastructures. Highlights include the Rangda heart-world which is a hybrid of O'Neil cylinder and Ring World. The system is defended by multiple War-Moons and other stranger weapons. Eddard starts working on building Ordinatus Rangda, (the redacted thing in the Night Lords profile) which is Four Blackstone fortresses jury-rigged by Mars into a system-killing distort weapon.
The Seven legions facing the Rangda fight a grueling war of attrition and suffer over a million Astartes casualties all told. Tyric makes a habit of hunting House-Lords and claims many new trophies. Captured Imperial data informs the Rangda of the location of Ordinatus Rangda. Lehyak-Prime one of the more powerful Rangda House-Lords leads an attack on Xana where it is being constructed. The data is a trick by the Ghost Legion and leads the Rangda into a trap where House Lehyak is almost entirely annihilated by the Imperium. Seizing the initiative the seven Legions start pushing deeper into Rangda territory and make it to Rangda Prime.
Multiple attacks against the system fail. Kalib Kraad is almost killed in one of these sieges and the Imperium is forced to blockade the system. The Xenocide against the Rangda continues. The arrival of the Emperor and Crusader Fleet Zero gives the Imperium the breathing room to make a full attack on Rangda Prime. The Ordinatus Rangda is used to destroy multiple War-Moons and breach the defenses of the Worm-World/Rangda Prime. (In canon a shackled Void Dragon was used for this.)
The Emperor faces the remaining House-Monarchs and the Life-Stream which is the insane bio-mechanical A.S.I. that is the spiritual leader of the Rangda. Here the Emperor reveals secrets pulled from the Hrud and Aeldari. The Rangda is a myth, they were once the K'nib and were the Old Ones secret police and enforcers. Who rivaled the Aeldari for succeeding the Old Ones. The K'nib were following the orders of some of the last Old Ones who were basically insane at that point and wanted to wipe out everything in the galaxy the Enslavers hadn't already killed. Planning to archive all the genetic and psychic material and wait for the Old One's return. The Aeldari stopped them and managed to breach the Life-Stream, and altered the species' collective memory.
Erasing the truth of them and putting the newborn Rangda to use as guards in the galactic north. The Tyranids make passes through galaxies every 50million years or so but can be fought off. The Old Ones and Aeldari did so in the past and the Rangda were left as an early warning system and the first line of defense. This information messes with the Life-Stream and lets Eddard kill it. Rangda Prime is scoured of all life and the Blackstone Fortresses are used to trigger a supernova, ensuring the system and anything hidden is destroyed. Eddard continues the war and spends the next two decades exterminating the fractured but still dangerous Rangda.
The 1st Legion encounters a Rangda monitoring post and the Imperium truly learns of the Tyranids, who are leaving an unknown galaxy that is lightyears below the galactic plane. The swarm is larger than the entire Milky Way and is looking for its next meal. It is not targeting the Galaxy yet, but will eventually. The Black Knights and Ghost Legion under their Primarchs' command start scavenging Rangda technology and preparing for the Great Devourer.