61: Chapter 61 Where is the third key hidden? The Overlord pieces together all the clues.

When the second key was loaded into the relic-grade containment pod, the Brood Queen's quantum-state translator was silently scrolling the latest data sent by the Overmind on the main screen of the briefing room aboard the Wrath of the Emperor. Marcus Aurelian sat at one end of the long table, the sling on his right arm finally removed, leaving only a faint calcification mark where the bone had fractured. Holding a cup of cold coffee in his left hand, he stared at the star map on the screen—a cross-referenced result compiled by the Overmind from the Necron Dynasty's records, the Old Ones' archived remnants, and The Swarm's own blackstone network scanning data. The coffee cup sat on the table, untouched.

The sealing frequencies of all three keys were now on record. The first had been duplicated by Tzeentch and was now void, though the original resonance characteristics of the Old Ones could still be extracted. The second was intact, its original sealing construction kit recompiled by the Brood Queen. The Overmind performed a quantum-state cross-comparison of the sealing frequencies of the two keys and discovered they both embedded the same ancient coordinates—not the location of the keys themselves, but the manufacturing site pointed to by the full set of design blueprints for the sealing devices. The third key was not in any tomb, not in any Necron Dynasty ruins, nor on any Chaos-tainted space hulk. It had never been hidden. When the Necron Dynasty dismantled the Void Throat, they left the third key at the sealing device's original manufacturing site—the Old Ones' last functioning creation laboratory in the Milky Way. That laboratory had existed within the territory of the Imperium of Man for ten thousand years, and the Adeptus Mechanicus had mistaken it for an ordinary blackstone relic mine. Its Imperial designation was Sigma-9.

The briefing room was so quiet that the faint hum of the quantum-state text on the main screen could be heard. Bern was the first to speak, his psionic talisman lying quietly in his palm on the table, its silver light faint but steady. "Sigma-9. That mine is on the northern edge of the Ultima Segmentum, less than two sectors' voyage from the Cadian Ruins—Tzeentch's last simulated resonance infiltrated from that direction. If the third key has been buried there all along, Chaos likely knew of its existence before we did." Sebastian took off his helmet and placed it on his knees. His Terminator assault platoon had suffered zero casualties during the tomb operation, but the continuous high-intensity orbital drops made his eye sockets look deeper than usual. "The Adeptus Mechanicus has been mining blackstone at that site for thousands of years. What did they dig up? The key, the sealing blueprints, or parts of the Void Throat? Those priests probably don't even know what kind of relic they're sitting on."

Marcus pressed the fleet channel, his voice raspy but every word crystal clear: "Oldenburg, find out who the current garrison Commander of the Sigma-9 mine is." Oldenburg's reply came back a few seconds later, a thin layer of anger suppressed in his voice: "There is no garrison. The Sigma-9 mine was raided three times by Chaos pirates after the fall of Cadia, and the Adeptus Mechanicus withdrew all Skitarii several months ago. Now, only the automated equipment at the bottom of the mine is still running on that planet, along with a small team of tech-priest explorers who refused to evacuate. Their last communication was sent a few days ago, containing only one line—'The vein is singing'."

Everyone in the briefing room changed expression simultaneously. The phrase "the vein is singing" had a specific origin within the Inquisition's archives on Chaos—when a large number of inactive blackstone engines are triggered by an external resonance source, creating Warp frequency coupling, the quantum lattice inside the blackstone vein emits a low-frequency resonance described by Psykers as "singing." In the final days before the fall of Cadia, all blackstone veins deep beneath the surface of Cadia began "singing" at once. Now, Sigma-9 was singing the same song.

The Overmind's quantum-state text updated on the main screen, cold green letters piercing the dim light of the briefing room: "Talasta Void Gate, Silent Tomb Old One sealing kit, Brood Queen inheritance authorization—cross-comparison of the three data sets complete. The third key is not a key; it is the master control key for the Old Ones' creation laboratory. The biological individual or civilization holding it can recompile the Void Throat seal itself. If Tzeentch gets it before we do, His failure in duplicating the first key will be compensated for by this master control key. He couldn't open the latch before, but the master control key will allow Him to dismantle the entire door directly."

The Brood Queen's translator lit up simultaneously; she had just returned to Metallis from the calibration mission for the second key's sealing kit, and the creep recorder at the tip of her forelimb was still connected to the quantum-state feedback line of the blackstone engine array. "Reiterate: The third key can dismantle the seal. Marcus, what do you plan to do?"

Marcus stood up, his fingers pressing against the edge of the star map. His bionic eye quickly scanned the relative positions of the Sigma-9 mine, the Cadian Ruins, and the sector where Tzeentch had last infiltrated, deducing that this was a narrow corridor; whichever side seized control first could use the third key as a weapon to block all other opponents. "The War Council authorizes a lightning strike on the Sigma-9 mine. Objective: Seize and safely transfer the Void Throat master control key before Tzeentch. Captain Sebastian will continue to command the assault formation, and the Brood Queen will remain the technical lead of the coalition. Bern, after the assault succeeds, take the Grey Knight squad to perform holy purification on all remaining blackstone engines in the mine. It has been confirmed that the singing blackstone is triggered by external resonance; if the source is not purified, they will keep singing until the Warp is torn apart."

He turned to Oldenburg and began deploying fleet movements. "The Third Assault Fleet will handle the entire escort. Nephilak will temporarily act as the proxy for the Griffin IV defense zone, and Canoptek-7 will be responsible for full quantum-state intelligence synchronization with the Eternal Echo. Once the troops depart, I will send the final briefing to Terra. You don't need to guess the contents of the briefing—I will tell the Council of High Lords that the coalition's final key assault operation has begun. As for why humanity spent ten thousand years standing on a sealing device in their own mine without realizing it, I'll answer them after I've won this battle. Oldenburg."

Marcus lowered his right hand, which had been in a sling for many days and had just had the brace removed, and pushed open the briefing room hatch without looking back. "Move out."