I try to approach this generously, but I can't help but think anyone thinking Khayon is "Mary Sue"-ish is either operating from misconceptions about the setting, or just not thinking things through clearly. (Although let's not rule out unclear writing. Heh.)
I mean, yes, he makes Magnus kneel... somehow. How do you really think that will happen, though? There's literally no way he can just out-cast Magnus the Red, so it's obviously not that. So people using this as an example of him being overpowered always sort of surprise me, since... I mean, being a Daemon Primarch isn't good thing. It's not a promotion. The Path to Glory doesn't actually end in Glory... I mean, people understand that, by and large. They don't in the setting, but readers usually do. No one can say with a straight face that Chaos characters are completely right and aren't massively deluded. Khayon included. (Khayon arguably more than most; several characters spend significant time telling him he's deluded about X and Y, even as he's telling them they're deluded about A and B.)
Daemon Primarchs are the Genie in Aladdin: "PHENOMENAL COSMIC POWER... itty bitty living space." It comes with huge drawbacks-- That's why it's a tragedy, it's why their stories are deceptions and tragedies. By the time they realise what they've walked into, it's too late. It's why Chaos needs mortal champions. It's why Abaddon is Warmaster of Chaos. So it's not a case of "But primarchs are the bestest" and that's that. They have staggeringly vast spiritual and metaphysical weaknesses, because becoming a Daemon Prince doesn't just mean you get better at everything, The End. And, well, to even wonder how one might humiliate a primarch is to forget the vaaaaassssst resources the Black Legion commands compared to the other Legions. Sometimes I get confused as to why the question is "How does Abaddon/Khayon/the Black Legion do X?" Well, because they can. They can, and no one else could. That's the point. Not because "ADB wants them to be the best." I couldn't care less who's "the best". It's just because this is who they are and what they do.
But Khayon, specifically:
Firstly, yes, he's one of the most powerful Chaos Space Marines in the setting. That's not because I sat there and thought "Oh, I'll make him great because lol." It's because, well, that's what one of the most powerful warlords in the most powerful Legion would be. If he was anything but that, it wouldn't be true to the setting or a character in that role. He has armies... just like every Black Legion warlord, except his are significantly smaller than most others. He's a pre-eminent sorcerer, because Abaddon famously has some of the most powerful sorcerers serving him, and it's alluded that Khayon may or may not be one of those famous ones operating under his own name instead.
Secondly, he never really achieves anything that isn't comparable to other characters in his position-- and often significantly less. What he achieves at the end of Talon for example, with sorcery and a warship, could have been done with a tractor beam or... heck, just a ship with its engines on, on a suicide run. Instead, Khayon does it to get a finer sense of control on the situation, and to do so, he can't move for half a year, has to be guarded at all times, has to be fed and given water like an invalid... and is weak for months afterwards. Look at a lot of the world-breaking sorcery unleashed by famous Chaos Marine sorcerers elsewhere, and you're honestly telling me that it's a big deal he guides an empty spaceship around for a while, at the cost of being an invalid for six months? That's a pretty massive downside. If that strikes you as impossible or unrealistically powerful-- especially in the Eye of Terror-- the issue isn't the event itself, it's, well, your perception of the setting. Is it a big deal? Yeah, kinda? But that's the point. These are the warlords of the Black Legion, foreseen by Abaddon as the potential future leaders of the Black Crusades. Even as Abaddon's assassin, look how many times he fails with Daravek due to... certain circumstances. Look at the effort and meticulous preparation that goes into his assassination attempt. He doesn't waltz into the palace and start lol-killing everyone because he's such a badass. He spends a year infiltrating minds slowly, inch by inch, and so on. He despises prophecy, which bites him the butt several times. He certainly can't predict what happens to Ahsur-Kai, when Ashur-Kai clicks almost at once. He loses ground against Moriana, who is one of Abaddon's faves. It goes on and on.
Thirdly, the fact he has esoteric bodyguards and a powerful weapon isn't a massive deal. It's not unique to him. It's not "I am the only one with these weird slaves." It's 'This is an example of the kinds of weird slaves Chaos Warlords have'. Khayon downplays Nefertari and co. consistently, citing how she'd last half a second on the battlefield, and how other Chaos big cheeses have equally weird champions. There was cool stuff in Ye Olde Lore about warbands having really wacky champions, and Khayon is just one example of that. (Telemachon and Vortigern are obviously hands-on and do it themselves, but I'm really looking forward to showing Amurael's warband champion in the next novel. Amurael has a very weird but understandable one, based on some old lore chats.)
Fourthly, look at what Khayon is actually good at, sorcery-wise. He's a daemonologist. That's his expertise. Something practically unseen pre-Heresy, but increasingly common in the Legions post-Heresy during their exile in the Eye. In many ways, that makes him the archetypal Chaos Sorcerer-- not a D&D mage in his own right, throwing fireballs on Ahriman's level, but a summoner par excellence. That's his speciality. He's good at other stuff too, but that's his speciality. So of course there's a lot of detail about the daemons he binds, and of course they're powerful.
I'm not saying these guys aren't big deals and don't do some serious stuff. They are, and they do. If they didn't, it would be laughable-- These are the characters leading the largest Chaos Legion and at the vanguard of the biggest incursions into the Imperium. That's the point. But this is one of those cases where it's not a particularly well considered criticism (which the words "Mary Sue" are often a great indicator of anyway) to say Khayon is overpowered, or whatever. Like... Think it through, honestly. Half the time I'm worried he's not massacring whole planets and turning them into Daemon Worlds, which is what a lot of these kinds of characters would actually be doing on their levels of power and influence.
Edited by A D-B, Yesterday, 09:37 PM.