Emperor's Sword
"...when the Evil Spirit assailed the creation of Good Truth, Good Thought and Fire intervened."
- – Yasht 13.77
The Emperor's Sword was used by Himself during the Great Crusade and by Primarch Roboute Guilliman as of the 42nd millenium.
The Wiki states thus: The Emperor's Sword, also called the Sword of the Emperor, is a famed relic blade that was wielded by the Emperor of Mankind Himself during the Great Crusade and was eventually passed on to the resurrected Primarch Roboute Guilliman in 999.M41 by the Archmagos Dominus Belisarius Cawl in the wake of the 13th Black Crusade and the Ultramar Campaign, just before he assumed the mantle of Lord Commander of the Imperium for the second time. The weapon had been kept safely in Cawl's possession on Mars for over 10,000 standard years, since the end of the Horus Heresy and Guilliman's first tenure as the Lord Commander. Touched by the Emperor's own psychic might, this finely wrought, master-crafted blade is lit from hilt to tip with leaping flames. When it is swung, the burning blade draws pyrotechnic arcs through the air, able to slice through the stoutest of armour with ease.
Obviously, this sword has a lot of meaning and importance for not just the Imperium at large, but also for many special characters, in particular in the books. When Dante is having prophetic visions during Devastation of Baal, he sees the Emperor in His throne without the sword, which freaks him out immensly, and kept pondering upon the implications of that vision for a while. This was resolved once the battle ended, after seeing Guilliman wielding the sword, and realizing the visions did not mean catastrophe, but the return of the Primarch. In Dark Imperium, Guilliman also reflects about the nature of the sword, and whether its properties are of divine or mundane nature, seeing as they are capable of banishing daemons even he would've struggled with if he hadn't had Daddy's sword. This, combined with his view on the Emperor's humanity-divinity (on top of all the shit he has to worry about in the book), makes him quite stressed out and melancholic of better times at the end of the book.
According to the sources, the blade is not only a powerful weapon in its own right, but it also has some of the most powerful cleansing abilities seen to date. Capable of dispelling Chaos energies and banishing daemons with ease, in point of fact, it can permakill them, annihilating their souls so they are no more, forever. This blade was probably made by the Emperor to specifically fight the malevolent forces of the Warp, and probably saw a lot of use during the War on the Webway, during the Horus Heresy. It also has a lot of similarities with the Spear of Russ, a weapon used by Leman Russ during the Battle of Trisolian that showed similar anti-chaos abilities, managing to de-corrupt Horus for a brief moment of time. Or more relevantly, Ghal-Maraz, the signature weapon of Emps' Fantasy Counterpart Sigmar, which has shown similar anti-chaos abilities to the point where it can convert Chaos Champions back to the side of order.
Considering how big of a McGuffin the Emperor's Sword is being in recent books, it's pretty safe to say Black Library probably has something prepared for it in the not-so-distant future.