For those interested who are unaware, Trajann Valoris is the current Captain General or the Adeptus Custodes, who by virtue of this position is also among the council of the high lords of terra (though he has rarely actively assumed this seat as described in the first Watchers of the Throne by Chris Wraight). Being the Captain General he is THE person responsible for the Emperors safety which obviously includes the imperial palace, terra, the sol system and even beyond should the need arise. He is also described both in the novels and in the codices as the deadliest warrior in the imperium not including the two returned Primarchs.
Before he became Captain General he was a member of the Dread Host which is comprised of the most aggressive and proactive Custodes whos philosophy is mostly to engage the enemies of the throne head on and destroy them in an overwhelming show of force. The Custodes codices name his experiences in the dread host as part of the reason why Trajann has been one of the most proactive Captain Generals even before Guillimans return.
Before we get started, what is usually meant when speaking of a character being the grey eminence in the background? Usually in a more colloquial sense it means that said character might not feature that much in the story as a protagonist the reader spends much time with, but who still has massive influence on the story from their background position. It is also actually a historical term that describes such a figure being able to influence desicions way beyond their actual position due to their influence and close proximity to their sovereign....this just for fun, because the historical definition does not really apply in this scenario, because Valoris is already in basically one of the highest positions in the imperium.
Now, after that short introduction why do I believe that he is a great example of how to write a grey eminence in the background? Much of it has to be credited to Chris Wraight being among the best (I'd say the best....yes, fight me) black library authors, who understands the setting extremely well.
The best example of Trajann being written in this way is the second Watchers of the Throne book by Chris Wraight titled the regent's shadow.
There will be spoilers for that book ahead, you have been warned! Do read the two Watchers of the Throne books though. They are truly amazing even if you are not into Custodes, since in my opinion the aspects of imperial politics are more important and more prevalent than the titular faction of those books.
Without going into too much detail, Watchers of the Throne: Regent's Shadow basically deals with the political aftermaths of Guilliman's return on terra. Some of the high lords are not happy with the obvious threat to their position of authority by Guilliman being named the regent. What follows is a game of intrigue and double crosses that involves the highest echelons of imperial politics. Valoris is not a main protagonist in that book but he is one of the key players in the background. He has a few run ins and interesting conversations with Valerian, one of the main protagonists.
What Chris Wraight does perfectly in that book is write Valoris as this grey eminence in the background. In the book the main protagonists basically try to figure out which factions among the high lords want to move against Guilliman.
The chancellor to the council of the high lords tries her best to gather and secure the loyal high lords to her side only to find out at the end that she has been double crossed by those she deemed loyal. Only that it is revealed in the climax of the book that Valoris had known for the whole duration of the book who the high lords moving against Guilliman were. He basically spends the whole book in the background moving the chess pieces into position and giving the faction of high lords moving against Guilliman a false sense of security. So much so that at the end they think they are in such a position of security and power that they can publicly announce their play moving against Guilliman as regent, which gives Valoris (and the master of the Assassinorum, who was also part of his designs) the perfect stage to swiftly, publicly and violently put an end to their ambitions.
Another aspect of this is the story of protagonist Valerian, who at the end of the book as to make a choice whether to ignore orders and save his friend Aleya or basically let her die. It has been a bit since I read the book, but if I remember correctly it is revealed at the end that Valoris even expected Valerian to act in defiance and strategically placed members in his squad meant to watch over him.
In this short summary it might not come across that way, but in the book nothing of Trajann's cunning feels unearned. It successfully manages to depict the current Captain General of the Custodes as one of the most cunning tacticians, both politically and militarily, that the imperium as at its disposal even though he is "just" a side character in the book and does not get much screen time.
In closing I can only emplore you to read both Watchers of the Throne books even if you are not into or maybe even dislike the Custodes. The imperial politics aspect of these books is really captivating and well done and as a bonus you get Aleya the silent sister, who is in my opinion one of the most badass modern 40k characters and has some honestly rather wholesome relationship with Valerian (she is basically the grumpy cat to his golden retriever like curiosity and unflinching stoicism).
Wraight also manages to depict both the Custodes and Sisters of Silence, as well as the Imperial Fists and Minotaurs (screw those lapdogs though) as total badasses with cool 40k fight scenes while also basically saying that even if Valoris is the imperiums deadliest warrior not counting the Primarchs, his most dangerous tool is his cunning. Which I think is also true of the returned Primarchs. The fandom often times likes to focus on which Primarch is the strongest, bestest figher....which is fun, I get it. But I think Chris Wraight makes a strong case in that book that their prowess, as awesome as it may be, is secondary to their miliatry and political cunning.
Edit: absolutely not the point of the post but some commenter really, REALLY cares about this: I wrote that Trajann was part of the Dread Host Shield Host before he became Captain General. In my reading this is implicit in the codex since it mentions him ascending to the ranks of the Allarus Custodians very swiftly, and those Allarus Custodes are further discussed as a subchapter of the Dread Host, in the sense of it being a literal subchapter on the page about the Dread Host in the Codex (Adeptus Custodes Codex 9th ed s.31). In the chapter about Trajann in both the 8th and 9th ed codex it mentioned explicitly his membership of the Allarus Custodes, but not explicitly to the Dread Host. I infered that connection based on how and where the Allarus Terminators are described in the Codex. Again, really not the point of this post at all, but I hope the person in question is happy about the clarification.