The Fall of Cadia happens 800-900 years after Horus Heresy, Cadia is a planet controlled by humans and is a very valuable planet cause Cadia is the birthplace of the Astra Militarum. Before the 13th Black Fleet arrived the forces of Chaos have launched some smaller crusade to Cadia stating from the 1st fleet that didn't do much to Cadia, Abadon the Despoiler one of Alpharius Omegons warlord's join the battle on the 13th fleet and bringing one world-destroying ship called the Black Stone Fortress. On the day the 13th Fleet arrived Cadia doesn't have time to prepare their defenses, they only need one day to put up their planet shields soo a Chapter Master from the Legion of Space Wolfs by the Chapter name of Sven Bloodhowl sacrifice himself and his entire battle barge ship to give Cadia one more day to live. 

After the sacrifice of Sven Bloodhowl the shield hasn't been put up yet, one person from the Adeptus Mechanicus tried to fix everything and then suddenly a very powerful energy burst entered the machine and turned on the shields right before The Black Stone Fortress opened fire. Sven Bloodhowl shouldn't have died that day but after cheer that the shield guarded Cadia Alpha Legion forces came and prepared for a ground assault, Cadia forces tried to fight off the Chaos enemy with the help from the Sisters of Battle and the Space marine but Abadon used the power of the warp and turned himself to a demon. in the middle of the battle a sacred fleet from the Adeptus Mechanicus arrived at Cadia and talked with the commanding officer by the name of Lord Castellan Ursarkar E. Creed, the person who talked to Lord Castellan was Archmagos Dominus Belisarius Cawl the leader of the Adeptus Mechanicus in Mars. Archmagos went to Cadia cause he saw the same technology that can stop the eye of terror (the eye of terror is warp filled energy that can kill or corrupt everything in its path) during his search he stumbles upon a common enemy called the Necrons (Necrons are ancient robots that came far before human and once control the entire galaxy), the Necron didn't fight Archmagos but what the Necron did was helping Archmagos cause they need the same technology Archmagos found.

In space, the Black Stone Fortress prepared their second strike until something came out from the warp gate(warp gate/ warp travel is used by humans and chaos to teleport to other planet's faster and guided by the emperor of mankind) what came out from the warp gate was the Phalanx (the Phalanx is a big fortress-monastery created by the Imperial Fists Space Marine chapter) and the entire Astra Militarum fleet with many help from the remaining fleet of Sven Bloodhowl and other chapters of the Space Marine. At that time the battle was chaotic and unpredictable until the captain of the Phalanx Tor Garadon decided to ram the Phalanx to the Black Stone Fortress, in the end the Black Stone Fortress was destroyed and humanity sigh in relief and more support fleet arrived like the Battle Group Silver Dawn commanded by Admiral Venesca Catallia.

But humanity isn't saved yet they still have the powerful ship called the Vengeful Spirit commanded by Alpharius Omegons and the incoming magic power the Eye of Terror, Archmagos and the Necron turn the machine on and safely stop the Eye of Terror. Their last treat is the Vengeful Spirit ship, Battle Group Silver Dawn was the closest to the ship so Admiral Catallia send an all-in attack on the ship but that wasn't enough the Battle Group Silver Dawn was destroyed and only leaving the Admirals ship. out of nowhere a fleet suddenly arrived in front of Admiral Catallia's ship, in the last attempt of desperation she rams her ship to the fleet and only taking out one vesel while the rest of the chaos fleet doesn't seem to mind her. In the end she realizes that the chaos fleet she passed by was going to ram the Black Stone Fortress towards Cadia and making a chain explosion that will destroy Cadia with everyone in it, all she can do is just look and stare as the Black Stone Fortress landed on Cadia making a big explosion and splitting the planet into two. everyone who fought in the battle cant do anything only die or retreat, that's the day Cadia Fell. Humanity retreat with the help from the Necron and The Craftworld Eldar (Eldar are aliens who came before humanity and lived prosperously until an accident happen that destroy their world).

The next story'll tell you about all races in the Warhammer universe and a small part of their lore

here is a video of what happened that day created by Games Workshop

https://www.youtube.com/watch?v=C07IBfAUaXY