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Given we know the metaphysics of dire weapons, it is safe to infer that the emperor's sword functions similarly.

Recall that he is a psyker Gestalt, he has more than enough soul to embed some of it into the sword.

Tldr

Dire swords have Eldar souls inside them. It has been shown that a potent soul can unmake a daemon (tethesis in the blade of asur).

The emperor's sword is a mega dire sword with a fragment of his soul embedded in it which does the same thing.

Hellebore wrote:


They also never really explain what about the emperor makes him capable of doing this - everything we know about him shows him to be an immortal gestalt psyker, nothing there that says why he can unmake daemons...

When things die in 40k, their souls enter the warp. Some are picked up by their patron god.. Even if that patronage is unwilling. I.E. Slaanesh has dibs on all Eldar souls, any Orks killed are grabbed by Gork or possibly Mork. SoB are absorbed by whatever the Emperor is in the warp. Most of the time you go to whatever deity you worshiped or pledged to. Those who didn't seem to just get tossed in until something picks them up and tortures them forever, 'cuz grim dark.

There are some exceptions of course. It seems that particularly malignant actions can cause souls to not go where they belong. I.E. a particularly powerful greater demon might rip out the soul of a SoB and keep it as a plaything, preventing it from joining.. Er, whatever it's supposed to.

But beyond that, souls seem to be extremely hard to destroy. There's certainly references to them being tortured while still in their living body - I.E. the Soulblaze spell literally setting your soul on fire - but we don't see many of them destroyed. This is why demons are immortal in the material realm. All you do is destroy the vessel they're wearing / whatever physical manifestation they've come up with. Their soul, I.E. the actual demon part, is just sent back to the warp.

Why is this important? Well...

The Emperor killed Horus by obliterating his soul.

It was stated that he did this to prevent Horus from ever coming back / being brought back by the Chaos Gods. So it appears the a psyker of sufficient power can straight up destroy souls. That makes sense in the context, given what we know about psykers.

As a result, it stands to reason that some weapons would be capable of literally destroying souls. This would probably be those with some sort of ultra-power psyker resonance. It's more-or-less the same concept as Force Weapons. While Force Weapons was technically a gameplay rule, the explanation (correct me if I'm wrong) was that the psyker was using their weapon to literally force their target's soul out of their body, regardless of how physically strong they might have been.

Force weapons in 2nd edition had improved Strength based on the psychic mastery level of the wielder, and could store Force cards, in effect acting like psychic batteries. The enhanced strength was supposedly from the psyker channeling warp energy through it. In later editions they changed the mechanics, though Eldar Singing Spears had a remnant of that with their Sx3 against vehicle armor values.

Voss wrote:

 ikeulhu wrote:
Its not just a matter of power, but of the source of said power. It is known that The Emperor is Anathema to the Primordial Annihilator, so it does make sense that imbuing His own power into a weapon could theoretically pass on that quality to said weapon. There is also the question of know how, perhaps multiple Grey Knights could harness the needed power to replicate the same effect, but that does not mean that they know how to properly do so.

Another possibility is that the Sword acts as a channel for the Emperor's energies, even while he is on the throne. This would allow it to perma-kill others regardless of who was wielding it as long as the Emperor persists, or maybe the link between Emperor and a Primarch is required for such a connection to be utilized.

The metaphysics in 40k does have actual rules that follow occult philosophies, and we may not know all those rules (such as exactly Why the Emperor is Anathema to Chaos) but they do exist. Personally, I like a bit of mystery, as things often can become less interesting the more they are explained.

'It is known' isn't an explanation either, nor is the chain of supposition that follows. What makes you think its even true?
Seriously, page reference in an existing book, because that's a pretty wild claim and other people are talking about off-the-shelf dire swords.

Chaos refers to the emperor by the name "The Anathema, we dunno what it means but the most accepted theory is that he is, literally, Anathema to chaos.

 Tyzarion_Kronius wrote:
I think Fulgrim getting killed would be a fitting move by Guilliman. Avenging Manus that way. I think it could make for a decent story where Guilliman severs Fulgrim's head with the Emperor's sword and brings his hacked-to-pieces-snake-primarch-body to Terra for parading.

Yeah, Guilliman seems like the kind of square who has never seen a horror movie. 'Its dead, now lets bring it to a populated area and turn our backs on it, even for a second'

 =Angel= wrote:

 Tyzarion_Kronius wrote:
I think Fulgrim getting killed would be a fitting move by Guilliman. Avenging Manus that way. I think it could make for a decent story where Guilliman severs Fulgrim's head with the Emperor's sword and brings his hacked-to-pieces-snake-primarch-body to Terra for parading.

Yeah, Guilliman seems like the kind of square who has never seen a horror movie. 'Its dead, now lets bring it to a populated area and turn our backs on it, even for a second'

It would cheer people up. But they should just make a copy of the head and throw the real one into a sun that nobody uses.

They need to get this sword to Kaldor Draigo stat. Daemon problem: solved.