🎶 Put the music on ! 🎤

Humanity was colonising the stars during the 30,000th millennium.  They were led by a guy called the emperor who had 18 sons (technically 19, 20 or 21), each leading one of the 18 space marine legions, space marines being augmented humans that are practically demi-gods. The space marines in the legions were seen as being the sons of their primarch. One such legion was 'The Emperor's Children', led by the primarch Fulgrim. One, semi-important member being Rylanor, who suffered severe injuries resulting in him being involved in a dreadnought. This meant his near-death body was piloting a massive mech. He was given the rank of "ancient of rites" and would help initiates learn the culture of the Emperor's children.

The emperor stopped leading the great crusade and gave the rank of warmaster to Horus, his favoured son. Horus then suffered an injury leading to him joining chaos, which basically compromises of the worshipers of 4 major gods. The greatest honour for chaos worshipers is to be ascended into a Daemon Prince, basically meaning they can't be permanently killed and have a bunch of mutations.

Horus convinced half the primarchs to turn traitor with him, one of them being Fulgrim. The Emperor's Children, along with the sons of horus, death guard and world eaters legions went to a planet known as Istvann 3. The parts of these legions loyal to the emperor (or suspected to be more likely to be loyal to him) were sent down onto the surface. Rylanor was not meant to be on the surface, due to a loyalist being a bit suspicious of the primarchs, swapped roles and went to the surface. Said loyalist found out the traitors planned on using virus bombs to kill all the loyalists on the surface. He flew down in an aircraft and warned them leading to most surviving it, although the city they were besieging was mostly ruined. The loyalists then set up a defensive leading to months of the traitors besieging them, towards the end of which Ryalnor was sent into a basement where there was a virus bomb. Most of the traitor primarchs that survived the entire following war were ascended into daemonhood, becoming known as the daemon primarchs, Fulgrim among them.

Around 10,000 millenia later he managed to make a communicator sending the message "the ancient awaits" as a lure to kill fulgrim. A group of thousand sons space marines, one of the traitor legions, were sent to investigate by their primarch. It then basically follows the song, just not as a song.

Most of that is extremely important lore or something (the entire first few paragraphs) but Ryalnor basically does not matter in the grand scheme of things. Fulgrim will probably return into lore in around 1-2 years in real life.