The Jailer obtained the plans from his mind and presumably used them to make Shalamourne. In the cutscene as he gets those plans from the Runecarver he shows what they were used to make, Frostmourne. He doesn’t say the Runecarver made the blade and helm, only that he created the plans to make them.
He is showing an image of Frostmourne, not the sword itself. His escape takes place AFTER the fall of Arthas because in the very first cinematics of him, he is still chained up. So the sword was made by someone else.
See even the title of this video is misleading… look at Frostmourne yourself… hes clearly not holding the literaly blade but an astral form of it. A Schematic
just search the forums. tons of people keep saying “The Jailer created frostmourne” Even the video above is claiming he obtained the item when he never had the physical item.
He probably wants to use Anduin to lead his armies into reality… his “Knight” needs the appropriate gear for said task.
Maybe the reason Arthas was chosen wasnt because of his position but maybe being strong with the light makes you a good conduit for these items… similar to how Ashbringer was easily flipped over.
The Shadowlands exist outside of “reality” same as The Twisting Nether and The Emerald Dream.
The Maw was created by the Etenral ones; the reason it is inescapable is because of The Winter Queen… if The Runecarver was/is the Primus then he’s possibly who created The Jailer chains and other items such as.
What Devos said was the runes on his blade were ‘unmistakeable’, and that Arthas was running around “with the power of the Maw in hand”.
I always found that sort of odd. How did she, as a Paragon, know what sort of thing was going on in the Maw? She could never have been there, never have seen anything from the Maw because as far as they knew then, once in the Maw nothing could get back out. So nobody in the Maw could pop back out say “ook there were these runes, look at this, I drew a few”.
So where would she have even seen the runes on Frostmourne before?
It makes me wonder if, from that brief touch she had of Uther’s damaged soul, that had been touched by Frostmourne, she was tainted even then.