It's established in the RH Space Wolves entry that their Primarch Leman Russ didn't die, but was sent somewhere as a result of a Necron weapon during the Sixth and First Legions' quest to the Necron Tomb World near the Maelstrom. The end of the Space Wolves' chapter hints that Leman Russ is going to find his way out of wherever he had been sent to in time for the fabled 'Wolftime' that he foresaw, so that's a given.
But then the Index Astartes Characters Annex dropped an interesting twist in the character of 'Prophet of the Wolftime', who is described as a daemon, the "Warp-born projection of the Space Wolves’ own desires and beliefs, of their quest for their lost Primarch", which is driven to pave the way for the Wolf King's imminent return to the Galaxy from wherever he had been sent to. The Gods are apparently calling for his return to the board, but then, the entry notes, even they don't know just what exactly will return from the place he was banished to. No one does except for the Prophet.
So some sort of substantial alteration had occurred in Leman Russ, and whatever he return as, it would likely not be as his original mundane Primarch self. So here then is the key question: What exactly has Leman Russ become? What will he return as?
Keeping with the apocalyptic theme of the entry, the concept of the Wolftime as some sort of Final Battle that would decide Mankind's fate, the current 'Times of Ending', the repeated emphasis by the splintered Space Wolves Legion's beliefs of him as 'the Great Wolf', the overall Viking theme of the Legion and other factors and foreshadowing, this troper believes that when Leman Russ returns, he would return transfigured into some sort of Beast of the Apocalypse. The resemblance of the fabled 'Wolftime' with the Vikings' beliefs of the apocalyptic 'Ragnarok' is unmistakable, and given their constant struggle against 'the Beast' inside themselves and their obsession with keeping Humankind pure, even though their very nature is corrupted, it seems supremely appropriate, ironic and altogether likely that the 'Great Wolf' himself had become one in the course of his banishment, especially if the realm he's banished to is some sort of Warp-based pocket-reality, meaning the beliefs of his Space Wolves over the course of ten thousand years and the corrupting effects of the Warp would end up turning him into something entirely inhuman, and inhumanly powerful.
So Bjorn and the other Space Wolves looking for him might finally achieve their goal later down the line and expect to find unchanged and still recognizable Primarch Leman Russ returning... and ended up getting RH-verse Fenrir instead.
We know very little about the Yellow King other than it is an absolutely nightmarish entity and that multiple characters have described it as "it is not what we think it is." The generally accepted (and thus wrong) understanding of the Yellow King is that it is just another godlike entity in the Warp... just worse than all the rest, ergo it has to be something else entirely.
There are few other clues, but together they support the theory that the Yellow King is an Enemy Without of the Emperor, all of his worst aspects cast off for the sake of guiding humanity to a better future. To whit:
- In the RH-universe, the Emperor is an Adaptational Nice Guy who is much more reasonable and accommodating with his sons, with nothing indicating that he cast off parts of himself that would be perceived as potential causes for weakness.
- In spite of Omegon's plan for the Emperor's ascension, the concept of the Starchild is never once discussed in-universe. No-one talks of the Emperor's soul such that it is fractured and needing to be reunited.
- Lorgar is implied in the Terran Crucible to be aware of the Yellow King, regarding it as a force separate from the Ruinous Powers. After killing the Emperor he tries to explain to Omegon as well that things had to be this way, meaning he had understood from the outset things would play out as they had Omegon's plan gone off correctly.
- The Raven Guard Index features a sequence with Eisenhorn, ending with him resolving to never allow the Yellow King to be "born". Thus whatever the Yellow King currently exists as, it can be safe to assume it is not complete in some form.
- The Endgame interlude depicts a card game between two figures, cloaked and only differentiated by the colors associated with them. While the figure representing Chaos only wears black, he has nothing that would associate him with any of the Chaos gods in particular and is instead simply a mirror of the other figure, who is understandably representative of the Emperor.
- While the Yellow King's name is obviously a reference to Hastur, at the same time yellow and gold are also the colors typically associated with the Emperor as well.
This is why the Emperor chooses death. To exist as a powerful and godlike warp entity would lead to reunion with the very parts of him that he wished to be free of, which would likely lead to very bad things happening to humanity and the galaxy at large.
Confirmed partially. It's a being which represents the Emperor as his enemies (xenos and heretics) see him, spawning from a seed in the Warp planted when he rejected power of Tower of Babel at the advice of Ollanius Pius.
It was none other than Lotara Sarrin herself, the Space Marines were there just to make sure that she didn't hurt anyone in the Administratum outpost. Modern Legionaries find it hillarious because no Astartes would be scary enough compared to her.
- As Yellow King is a representation of Emperor's arrogance and callousness, which played a large role in failure of majority of his plans, so may Yellow King's.
- It's unknown what happened to Nemiel, and Tuchulcha didn't manage to reveal it, but there are implications that Nemiel could have survived, albeit corrupted or tortured into treachery.
- As his daemon Lurgon implies, The Death God knew or at least guessed that the Emperor planned to get himself killed in order to make Chaos Gods vulnerable. Nurgle is the lord of ruin and ending, so it's in his nature to support this. For example, his servants Castus and Parmemides, aided "Marius Gage" simply because Ultramarines prevented Roboute from dying.
- Judging by Endgame, the Yellow King manipulated and corrupted Primarchs, is responsible for Bile's Consortium, and many other evil deeds. While it may be because of his nature as the Emperor's dark side, he also put tokens of gods onto Primarchs, meaning he may very well be much more: a true personification of Chaos, as gods are merely part of the great horror that is Primordial Annihilator, and Warp doesn't have linear time...
- The reason that Erebus in RH is so sane and reasonable is that he never got killed and replaced by a power hungry cultist.
