There is only one Frostmourne. There shall never be another.
Now, how this idea could work is that DKs got access to mournblade transmogs. That would be a cool mini game, having to hunt them down… just like with warlock pet customisations.
The majority of rune blade transmogs lean into expansion specific aesthetics and don’t look like actual DK weapons.
In fact - Arthus was a paladin that was corrupted.
The first and greatest of these was the Lich King’s chosen champion, Prince [Arthas Menethil], once a mighty paladin of the [Silver Hand]
Wrong.
Upon entering one of the portals, Ner’zhul and his followers were immediately captured by Kil’jaeden. At the demon lord’s command, a group of dreadlords consisting of Tichondrius, Mal’Ganis, Balnazzar, Detheroc, and Varimathras tortured the orc shaman Ner’zhul after he had escaped from Draenor to the Twisting Nether. Eventually, Ner’zhul, his sanity cracking, would agree to serve Kil’jaeden, who passed the orc’s spirit through death and revived as a spectral entity. The dreadlords bound his disembodied spirit to a specifically crafted set of armor and a mighty runeblade called Frostmourne. These items were locked in a diamond-hard block of ice to imprison Ner’zhul.[21] The Lich King and the dreadlords who had tortured him were then sent to Azeroth in order to weaken any resistance that the Burning Legion would face during the Third War. While the Lich King gathered his strength, Tichondrius and his fellow dreadlords would serve as jailors and executors meant to hasten the Lich King’s task by any means necessary. To this end, they would construct the Icecrown Citadel within Northrend. Though seemingly broken by the dreadlords, Ner’zhul sought vengeance and was able to play on the dreadlord’s fears of Kil’jaeden in order trick them into allowing him to hurl Frostmourne out of the Frozen Throne.[22] In time, Ner’zhul would guide Arthas Menethil to Frostmourne in order to make the young paladin his champion.
I love you too buddy. 
The Orc Teron’gor (known as a Teron Gorefiend when he resurrected) a follower of Gul’Dan and the Shadow Council, was the first Death Knight, created to combat Human mages. His heart was cut out of his body and formed into a jewel used to imbue his spirit into a weapon that powered the corpse of a fallen Knight of Stormwind. He even tells you he was the first DK in Black Temple! " “I was the first, you know. For me, the wheel of death has spun many times. So much time has passed. I have a lot of catching up to do…” There were many orc Death Knights created by this method for Ogrim Doomhammer’s Horde long before Frostmourne was ever retrieved by Arthas in Northrend. You are arguing for it to be exclusive to Death Knights as its iconic weapon and you don’t even know the lore…like I said, it was the Lich Kings weapon, not a Death Knight weapon.
Read that again, Ner’zul being the Lich King and the dreeadlords were sent to Azeroth - Lich King = Ner’Zul
Yes, Ner’zhul was the first Lich King and arthas the first death knight.
If you want to consider Gul’dan shadow council warlock’s souls “implanted” into dead soldiers as death knights, you could call Arthas the first death knight of the lich king or the first death knight of the second generation.
Nevertheless, Arthas was a Death Knight and Frostmourne was his mighty weapon.
This is what matters, why we want the xmog.
I say it slow, Arthas. was. not. the. first. Death Knight. Teron Gorefiend, an Orc in Doomhammer’s Horde was.
Frostmourne was given to us in Legion though. And it worked the same way as the original Frostmourne, all it wanted was more souls.
It was created by Primus, enslaved by the Jailer into the runecarver. Zoval gave Frostmourne and the Helm of Domination to the dreadlords, who manipulated Kil’Jaeden into creating the Lich King.
But it was shattered and made into 2 1 handers which is not as cool a look as the Frostmourne appearance.
Careful: if you keep using facts and logic then he’s going to ignore you and then demand that everyone report you for pointing out the problems with his position.
For example, he got real mad when I pointed out how dismissive he was to someone who suggested that warriors should get that sword anduin and Varian were swinging around because it was canonically in use, to which I pointed out that Frostmourne canonically was reforged.
He of course argued that this would be a different weapon entirely wholly severed from any of the things that made frostmourne anything of note to which I pointed out that it should therefore be available to everyone.
It’s a shame he can’t be reasonable.
Yep this is what you have been arguing against. Finally you wrote something that agrees.
I thought they looked better than the original Frostmourne. Especially their alternative look. The old Frostmourne was too low res.
Nah man nah.
We want the big stick, bro. Not the tooth-pickers.
Its a shame he doesnt understand that DK already took Frostmourne and reforged it into the frost DK artifact, and that any new weapon model creeated in gamee should be accessible to all classes and not DK specific.
haha 
fair enough.
They can make some lore reason to get the appearance again if they wanted to.