Malum Caedo (Latin for "I kill evil")

Several standard years after the events of the Ork invasion of GrimSkull and Titus's Crusade against Chaos on the vital Forge World of Graia, Starguard veteran Malum Caedo and several of his brothers of the Ultramarines Chapter's 1st Veteran Company were sent by the Inquisition back to Graia on a mission of great importance.

Ever since the Ork and Chaos invasions of Graia that were unleashed on the Forge World by the machinations of the Chaos Sorcerer Nemeroth several standard years earlier, the Inquisition has kept Graia closely watched for the emergence of new threats. Inquisitor Seibel of the Ordo Malleus revealed that rogue Tech-priests from Graia had been experimenting with the remains of the late Inquisitor Drogan's unique and unstable Warp-based power source - a small cylinder encased in potent blue energies - and the experiments had resulted in the opening of a Warp rift that allowed the forces of Chaos to once again invade the Forge World. Malum Caedo and his Veteran Starguard Squadron were tasked with recovering the power source with the help of a Skull Servant that once belonged to Drogan.

When the Ultramarines made their planetfall, their Drop Pod was severely damaged during the descent and crashed onto the planet's surface, leaving Malum Caedo as the sole survivor. While Caedo fought valiantly alone against the forces of Chaos, including the Chaos Cultists, the Heretical Astartes, and the Daemons, it was discovered that the leader of the Chaos incursion, a Chaos Sorcerer named Tumulus Samael of the Black Legion, sought the power source for his own nefarious plans to transform Graia into a Daemon World under his control and the orbital battle fortress of the Graian Crown into a Warp-based weapon. The sorcerer had already managed to discover the location of the fragment of the energy source and Caedo rushed to stop him.

When Caedo reached the Adeptus Mechanicus vault containing the power source, Samael appeared and boasted

having already taken the energy source. The sorcerer and his minions attacked the Void orbital station above Graia and used its power source to increase the Warp rift that threatened to engulf Graia and the entire sector.

Malum fought his way through the void station before finally reaching Samael, and after a grueling fight, the Starguard Veteran managed to kill the sorcerer. Inquisitor Seibel arrived at the empty station shortly after and retrieved the power source for the Inquisition. Without the power source, the Warp rift began to close, but Seibel warned that there were still many Chaos forces active in Graia, and Malum Caedo prepared himself for the trials that lay ahead.

Malum Caedo continues to serve in the Empire as a Space-Marine of the First Company of the Ultramarines, being the new Chief Welder of the new Starguard Recruits.

(The Star Guard are the most elite soldiers within the First Company. They are the best prepared Space Marines in the entire chapter, the Best of the Best)

The Space-Marines or also known as Adeptus Astartes are superhumans generically modified to be stronger, more agile, faster and much larger than an ordinary human. The Space-Marines also have their leaders, the Chapter Masters, being the Master of the Ultramarines Marneus Calgar.

However, some Chapters have as their leader one of the Primarchs, the Sons of the Emperor himself, and the Ultramarines serve Roboute Guilliman, the Founder, creator and father of the Ultramarines, thus Marneus Calgar is the second in command behind Guilliman.

And one of the main allies of the Space Marines is precisely the Astra militarum

The Imperial Guards of the Astra militarum are a military force made up of ordinary men and women of the Empire. Unlike the Space Marines, the Imperial Guard fights using far inferior weapons and equipment, but to compensate for this, the number of soldiers on the battlefield is Millions to Billions of soldiers with Tanks.

true support artillery with guerrilla and trench strategies the Imperial Guard of the Astra militarum is divided into regiments each regiment belonging to different planets which makes the Imperial Guard as a whole quite unstandardized among themselves since each regiment is unique even the majority of the Regiments follow the Cadian battle standards since the Cadian Regiment is considered the best Regiment of soldiers of the Imperial Guard

Thus creating a very big difference between a Space Marine and a Guardsman.

Even though Space-Marines are genetic heirs of the Emperor himself, they are treated and revered as Demigods on the battlefield, with all Guardsmen who encounter them kneeling just for this in the presence of an Astartes.