"The new Scouts fared well?"
It was the first time that the Grandmaster had spoken, since acknowledging Artur's arrival and their bout had commenced. As ever, it was impossible to read his unblinking, sightless eyes. So like the Mantis itself, thought the honoured veteran Commander, encased in his metal shell. He cast his gaze across the board once more. Resignation, perhaps? The servos had by now relayed all permutations to the Grandmaster, and the pincer of Artur's remaining Astartes assured him victory against the ancient warrior. A ploy? Perhaps. But with only the Rex remaining, his choices were few, his options limited beyond noble self-sacrifice.
"Aye, my lord, well enough. They will be fine instruments of the Emperor's will, should he deem them worthy." He shifted his remaining Devastator to pin down the gleaming, golden Rex. "You are checked, my lord. And I believe the game is mine."
A statement, not a question.
"Tell me," continued the Grandmaster, barely acknowledging the inevitable, "Were they as attentive to all of the details of Tranquility as you once were?" A nudge from the servo-arm, prolonging the endgame. Artur frowned.
"I schooled them in full, just as you did me, sire." The towering Terminator shifted to his whim, "And checked once more, upon two fronts." The pause lengthened like a rift in time itself. "You have but one move, Grandmaster, and it ends in Regicide."
"Did you know that I was a Vanguard, Commander?"
Artur frowned despite himself, accessing memories deep in his core.
"My lord, there were no Vanguard Veterans of the Tranquility campaign."
As if from nowhere, the table shimmered, shifted.
"None that you know of, child." A rare crease at the corner of the Grandmaster's mouth - a suggestion of a smile?
A Vanguard, clad in corvid armour and streaked black on yellow, bore down upon Artur's Rex, and struck hard and true.
"A Mantis always guards his secrets, Commander. Regicide, I believe. As you say, I had but one move open to me."
There was nought he could do but let cry a wordless, furious yell, slamming his fist to the table and sending his slain Rex reeling.
"We are hidden even in the shadows of our own history, Artur. The treachery in the truth, the hiding in plain sight - the ancient art of the Mantis."
But his words were dust in the recycled air, as his Honoured opponent was long gone.

I'm fond of David Mamet, from whom the quote comes (I am an English (& Media) teacher, after all) and this spurred me on to thinking about Stanley Kubrick's 'The Killing' (I also teach film) with its iconic chess scene (great movie, great director, and cited by Mamet as one of his favourite moments in drama - yes, I am a nerd, thank you). I got thinking about chess games generally, and Paradigm mentioned Regicide, the 40k descendant of the game. And so it came together. The venerable Mantis General howling in rage and frustration at the unexpected defeat! I tried to stay true to Gitsplitta's interpretation of the MW, and afaik he's not done any Tranquillity pattern Vanguard Veterans so I factored that into the fluff