Remember the first line of the entire book series.
"I was there, the day Horus slew the Emperor."
We've always known this would be an ironic echo, but now, decades later, the truth of that comes to light.
Horus slew the Emperor, allowing his soul to reincarnate, and took his place on the Golden Throne, the final act of eternal penance for the Emperor's most loyal son.
Do it GW you cucks. The amount of SCREAMING would be epic. If you need some woke bullshit to make the pill go down better, let the Emperor reincarnate as a brown lesbian arabic jewish chick or something! YOU KNOW YOU WANT TO.
The Custodes aren't guarding the Emperor.
They're making sure Horus never leaves that throne.
That's why no one's allowed to see him. That's why Cypher was turned away -- can't have him killing Horus, even if it would redeem the Dark Angels in their eyes. That's why Guilliman was so shook by the encounter, why the Emperor that spoke to him was nothing like the father he remembered.
Because it wasn't his father.
It's the secret Dorn took to his grave. They didn't entomb the Emperor on the golden throne. They entombed Horus Lupercal. The final irony of the Heresy. Horus gets the throne he felt he so richly deserved. Only now he gets to spend the rest of eternity in torment, holding the damaged webway portal closed, all while the person who SHOULD be sitting on the throne, the person who could easily shoulder the weight -- Magnus -- he personally arranged things so he'd be damned forever instead. AND caused the great work to be damaged to begin with.