From the writeup on WarCom, it sounds like it'll kind of be a spiritual successor to Angels of Darkness, in that it'll be a series of interrogations as a framing device to explore Luther as a character, his actions and decisions.
While we can extrapolate and interpret most of his overall actions during the Great Crusade and Heresy, and a bit before that, we never really got into his head. We never really had alone-time with him. Even Zahariel didn't really have "honest" chats about what was going on in the novels, and Astelan always manipulated and misrepresented everything to his favor.
We can see Luther being conflicted in many scenes across the Heresy series. Doubting himself, as much as the Lion or the Emperor. There's SO MUCH there that can be explored, and not in a simple plot, but throughout the various phases of his life: The miracle knight, the knightly lord, the father figure, the footnote in the Lion's shadow, the man seeking to free his people from the opressive nature of Caliban, just to tumble into a completely industrialized world rid of all the charms he cherished, a culture destroyed. A man who wondered if he made a mistake in saving Jonson, hesitating when faced with an opportunity. A man cast aside under the guise of honour. A man who wanted freedom from yet another system of oppression. A man who ended up learning too much of the lore of Caliban and the Warp, who saw yet another opportunity but did not wholly grasp it yet reject it until his hand was ultimately forced into treachery, with all other paths cut off.
There's so much to Luther as a character, little of which we've seen in detail - because even in the books, even as Zahariel becomes somewhat his protege, Luther himself was still always a distant figure on the sidelines, who had to hold up appearances.
I'm super duper stoked to see him be brutally honest in the cells of the Rock, to see the glimpses that shaped the man of honour and glory into one despised as an arch-traitor for not only rising up against the man he should have been the closest to, but also "killing" him before looking at his work in horror.
The more I think about Luther as a character, the more I am feeling like he might be one of the most compelling personalities in the entire fluff. And... he has a part to play yet, outside of the Rock.