I hope I get get a mourneblade from the Runecarver later on
We have to remember that Frostmourne was originally stated to be created by the Lich King. Only afterwards was credit given to the Nathrezim in the book Arthas: Rise of the Lich King. It could be that the Nathrezim stole the schematics and created it themselves.
Also, if you think about it, if Runecarver is indeed a Primus it would make sense why the vessels of Domination have control over Scourge. Since Primus is like a leader/head of Maldraxxus and Maldraxxus is to be sort of army/defense of Shadowlands, the vessels created by him would have control over the army. And Scourge as it was said recently originates from Maldraxxus.
Ever notice that all of Maldraxxus is similar to Naxxramas' wings?- Arachnid Quarter = House of Eyes uses primarily arachnid humanoids- Construct Quarter = House of Constructs - Military Quarter = House of the Chosen- Plague Quarter = House of Plagues We saw in the Afterlives: Maldraxxus video that the House of Plagues 'fell', while questing we learn that the House of Eyes was also destroyed. The House between these two, House of Rituals, is home to the Liches, and the final wing of Naxx is the Lich, Kel'thuzad. More quests reveal that a Baron from the House of the Chosen 'went missing' a long time ago, and the new Baron is in leagues with the Jailer. (Personal Speculation:) Perhaps the Jailer lured the Primus into Torghast to retrieve Baron Mograine where he was met with the shackles that bind him now. With the Primus gone, The House of Rituals (and any allies they could turn from the other Houses) were free to launch their attack on the house of Plagues to take... well the plague... the House of Eyes, being spies, retaliate forcing the liches to wipe them out as well. The liches give the plague to the Mal'ganis who brings it to Azeroth, setting into motion their plans to lure someone to the blade Frostmourne, revive Kel'thuzad into the living world, and put on the Helm of Domination, letting the Jailer escape his prison through the body of another. How long the dreadlords were involved and if Kel'thuzad was in the Shadowlands to oversee the assault are up in the air. Most of this is pure opinions of my own and not canon facts mind you.. :P
Seems we're all assuming this memory refers to the original Frostmourne and Helm crafting. Surely timeline wise it makes more sense that the jailer requires them made anew for his secret weapon. Firstly, how else could he know of them to request the runecarver make them? Secondly, as I mentioned it would fit the timeline far more snugly.