Probably the most iconic weapon out of the Warcraft franchise, wielded by the most iconic villain, who is also the highest-CR monster in the WoW D&D book, Frostmourne carries with it a certain caché. Ironically, though, the story is not nearly as complex or extensive as that of its opposite number, the Ashbringer (which I covered in the previous post).
It was only during the very controversial Shadowlands expansion (I say controversial - it might be better to say it was unpopular, though I maintain it is not the worst expansion, and was certainly better than Warlords of Draenor and possibly its predecessor, Battle for Azeroth) that we got hints as to the deeper history of Frostmourne.
Let's talk about the original lore, which will be quite quick, and then we can do the Shadowlands stuff (which is mentioned in the book).
So, when Arthas pursued the Dreadlord Mal'ganis to the frigid continent of Northrend, he struggled to slay the demon in a permanent (or at least longterm) manner. He had happened across his old dwarven friend Muradin Bronzebeard, one of King Magni's younger brothers. Muradin, pursuing the dwarven fascination with archaeology, had found stories about a powerful magic sword, and Arthas thought that this might be the key to defeating his demonic foe.
However, upon finding the blade, Muradin read a warning that had been left there: anyone who took up the blade would lose their very soul. Arthas, in an act of reckless wrath, shrugged off this curse and took up the blade. The tremors that followed from this act of doom would knock out Muradin with falling rubble, and the newly soulless Arthas left his friend for dead, keen on getting his revenge.
Claiming victory against Mal'ganis, Arthas returned to Lordaeron to a hero's welcome, but the sword had done its work - unbeknownst to the celebrating crowds in Lordaeron City, along with Arthas' father King Terenas, Arthas had not only become corrupted himself, but dragged his own forces into undeath with the power of the sword. Arthas stabbed his father through the neck with the blade, and began a massacre of the capital city, spreading the undead Scourge across the kingdom that he had technically just inherited as the new king.
The blade, it turned out, had come sealed away in the Frozen Throne along with the Lich King.
The original Lich King had been the soul of the Orc shaman Ner'zhul, who had originally been tricked by the demon Kil'jaeden into forming the Horde in its murderous original incarnation. Ner'zhul, seeing the Horde defeated in the Second War, fled back to Draenor, and used artifacts looted on Azeroth to open numerous portals to other worlds. The stresses tore the planet apart, leaving the surreal landscape of Outland behind, and Ner'zhul found himself captured by Kil'jaeden the moment he stepped through his chosen portal.
Kil'jaeden tortured Ner'zhul, tearing his soul from his body, and sealed it into a suit of armor. He then froze that armor and the blade Frostmourne into the icy crystal that would become the Frozen Throne, which he hurled toward Azeroth. It landed in Northrend, and from there, Ner'zhul worked to build an army of the dead on behalf of the demonic Burning Legion.
The blade bound Arthas' will to that of the Lich King, and removed all inhibitions from his latent sociopathic tendencies. While he furthered the Legion's plans during the Third War, he also secretly worked to undermine them, the Lich King's ultimate goal to break away from the Legion's control.
Arthas would go on to defend the Lich King even after the Third War from forces the Legion sent to wrest back control of the Scourge, and upon his victory, he ascended to the Frozen Throne and used Frostmourne to break the armor of the Lich King out of the crystal. Taking the Helm of Domination, the capstone of the Lich King armor, onto his own head, he merged with Ner'zhul, ultimately absorbing the Shaman and becoming the dominant will within the Lich King.
He actually spent several years frozen back into the Frozen Throne, commanding the Scourge mentally from his seat, and Frostmourne went unused during this time. But when his grand plan - to provoke the Alliance and Horde into sending him their greatest champions so that he could raise them as his own, he emerged from the Frozen Throne and took on a more active role leading the Scourge in person.
Frostmourne remained his weapon during this time, but when the final battle came to its culmination, as he sought to raise the greatest heroes of Azeroth as his own personal agents, Tirion Fordring, wielding the Ashbringer, shattered Frostmourne, leading to Arthas' demise and the defeat of the Scourge.
However, the shards of Frostmourne remained - later, during the last full-scale invasion by the Burning Legion, some Death Knights traveled to retrieve these shards, and had them reforged into a pair of shortswords called the Blades of the Fallen Prince (the artifact weapon for Frost Death Knights during Legion).
Now, you'll note that there's basically no actual explanation of where the blade even came from (and some questions, like how there were already myths and legends about Frostmourne that Muradin was chasing down when the sword could have only gotten to Northrend, like, a year or two earlier at most).
In the Shadowlands expansion, it was revealed that while Kil'jaeden had played an active role in the creation of the Lich King, the actual source of power behind the Scourge was not demonic/fel in nature, but was the power of the Shadowlands - Warcraft's realm of death (which includes the afterlives).
Frostmourne was actually one of a handful of Mournblades, which were created on behalf of the Jailer - an incredibly powerful entity of the Shadowlands whose malevolence saw him locked away in the ultimate planar Oubliette, the Maw - almost precisely Warcraft's version of a dark, bleak, cold hell.
The Jailer, with help from his prisoner, the Primus (a being of comparable power in the Shadowlands) produced the blades and the Helm of Domination as part of his plan to sweep over all of creation and bring every living thing under his direct control, like a master puppeteer for the entire cosmos. This power of domination is at the heart of evil necromancy, and was the primary tool of the Jailer.
So:
Given the lore - that the sword claims the souls of any who take it up or who are slain by it - you might think... this is probably not the safest weapon to wield. And you'd be right.
It's also one of two artifacts that are listed directly as equipment on the Lich King Arthas stat block (the other being the Helm of Domination). While the Helm does actually give you a chance to retain your free will if you put it on (much as Bolvar Fordragon, who became the Lich King after Arthas, did, and used his powers to hold back and weaken the Scourge,) Frostmourne doesn't work that way.
Upon attuning to the sword, you become Undead (if you weren't already) and your alignment becomes Neutral Evil (if it wasn't already). These changes remain in place even if you end your attunement.
Now, I think there's a debate to be had as to whether Arthas was already evil before he took up the blade, or if he was still Lawful Good but had just taken his convictions to an extreme that accomplished evil despite his intentions (it's a very dark thing to debate, but his infamous Culling of Stratholme... might that actually have been the right call, horrific as it was?)
Also, while attuned to the sword, the Lich King can see through your eyes and hear through your ears, and communicate telepathically with you regardless of distance, even across planes.
So, this raises an interesting question: if you slay Arthas and get this, who is the Lich King? In theory, it's whoever then loots the Helm of Domination. While I won't do the whole stats for that, attuning requires a DC 18 Charisma saving throw to avoid having your mind absorbed by the Lich King. Bolvar, who was probably a 20th-level paladin in D&D terms, might have had as much as a +15 to such saves (with Aura of Protection,) so it makes sense that he'd be able to succeed.
Ok, now the core things:
Frostmourne is a +3 Greatsword. It does 2d12 extra Cold damage on a hit. If you reduce a creature to 0 HP, the creature dies and its soul is trapped in the sword. While trapped, the target cannot be raised from the dead until Frostmourne is destroyed.
I'd assume "raised from the dead" means specifically resurrection magic, as you sure as hell can be raised as undead. That's kind of the Lich King's whole thing.
Now, while anyone can attune to Frostmourne, Paladins get additional benefits (interesting question: do we count Death Knight here? I think the precedent we have is that Doomhammer, a famous Shaman weapon - actually the Enhancement Shaman artifact in Legion - requires attunement by a Barbarian, not a "Shaman," so I'd say Paladin here includes Death Knights - which only makes sense). First off, Undead creatures of your choice (excluding you) in your Aura of Protection have advantage on attack rolls and saving throws. Then, also, all allies in your aura gain immunity to Cold damage.
I'd actually strongly consider integrating that second part into Arthas' stat block, as he has some powerful AoE abilities that would theoretically hit his undead allies and do frost damage.
Beyond that (oh, was that all?) there's 1 major beneficial property, 1 minor detrimental property, and 1 major detrimental property.
Major detrimental properties can be very nasty - the 97-100 one prevents you from regaining HP, for example.
Finally, Frostmourne can be destroyed if its wielder is slain by someone wielding the Ashbringer, and only the Runecarver (er, spoilers: aka the Primus) can reforge it from the fragments.
So:
Two things:
First off, chances are that if you have gotten Frostmourne, you've done it by defeating Arthas, which means that you're probably at the end of your campaign. Arthas is built as a campaign end-boss, and while I'd argue that on the strict "tiers of play" guidance, he really ought to be a tier 3 boss (as he's a global, rather than multiversal threat) I also get that he's the most popular Warcraft villain and they want him to be super powerful. (I'd argue that expansion final bosses who do warrant tier 4 are probably Kil'jaeden, Archimonde, Argus, the Jailer, and Dimensius the All-Devouring, and possibly N'zoth and C'thun (the latter of which retroactively became the final boss of Vanilla because of Naxxramas' relocation to Northrend)).
Technically you don't become an NPC under the DM's control, but you might want to talk about what it means for your character to become Neutral Evil. While you can certainly have an evil team player in a party, the narrative of Frostmourne is that you don't just turn evil, but become a villainous antagonist to the party.
It is cool that we get this weapon statted out. But at the same time, I just don't think it would be practical to ever have a player use it in a campaign - unless you want the finale of a campaign to be the rest of the party fighting against the one who has just attuned to it. Or, if you're playing a villainous campaign, sure, maybe then.