depends on the era. pre-horus Heresy, it would go to the emperor of mankind and his sons, the primachs.
current era, it's his son and current defacto leader of the imperium of man, roboute guilliman.
but you gotta understand. WH40k is not only really dark but there are a lot of different factions and groups that span the whole of the galaxy. some, doom could handle. others? doom would best stay away from.Okay, so probably not far enough before he has to rely on some serious prep to steal power a'la Beyonder? Is that a possibility here or is he SOL up to a certain point?The top dogs are probably the gods. You have your Chaos gods (Khorne, Slaanesh, Tzeentch, and Nurgle), then you have your old ones, your Eldar gods (Cegorach, Khaine, Isha, Asuryan, Ynnead, there might be some others I am forgetting), you also have your C'Tan (the outsider, the deceiver, the void dragon, and the burning one), the God Emperor of Mankind, the Machine God, and Gork and Mork. These are all beings that operate on at least a galactic scale.
"Sir, does this mean that Ann Margret's not coming?"
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"One of the maddening but beautiful things about comics is that you have to give characters a sense of change without changing them so much that they violate the essence of who they are." ~ Ann Nocenti, Chris Claremont's X-Men.
I doubt it's possible to power steal from the top of the food chain in 40k, not only are the chaos gods hideously powerful, they live in a universe that obeys them almost exclusively. Unless Doom wants to magitech up himself a way to be an Xavier level psychic, nothing he has matters in the Warp, the only relevant strength there is mental, and even then it tends to only make you more vulnerable to daemons, not less. He could try a C'tan star god or Farseer, but they're waaaaaaaay below the chaos gods and they'd still easily stamp him out if he stepped into their yard.
The Emperor was a powerful psychic tens of thousands of years old, it's not impossible but highly unlikely. Emps subjugated the Void Dragon and imprisoned it in Mars in the Middle Ages, created a sun inside the Imperial Webway, can summon the souls of anyone who ever died for him, and is regarded by the chaos gods as their "anathema," their opposite. He's a unique case, I wouldn't use him as a baseline for who can get one over on Chaos. Same goes for Kaldo (ugh) Draigo, not anywhere near as powerful but still able to wander the Warp freely and (relatively) safely, the nature of his power lets him impose his own will on the Warp around him. Also Draigo has Emp's own DNA grafted onto his own like all Grey Knights, probably doesn't hurt. Doom could pull off something similar, but it's not just a matter of will. 40k psykers can do things like crush gigantic mecha, hurl lightning and open dimensional rifts, but they're still nothing to the chaos gods.
True I just meant he is not generally portrayed as any more intelligent or competent than Doom. I might argue that Doom has better feats overall but a lot of that is due to the Emperor only being seen so often in lore. Baseline yeah the Emperor is WAY above Doom but that is where the power jacking comes in. Doom could likely crush a giant mech if he wanted (with his armor, not through mental powers) to and he can definitely open rifts. I just wonder if the crimson bands would even work in the 40k verse.
I get it, but it's more about the method rather than power level. 40K is very specific about psychics and their relationship to the Warp, magic is derived from it only, there is no other way to use "spells", and technology can only interact with it in VERY limited ways.