And to anyone other than a murderhobo, that'd be obvious.
Seriously, your player, OP, is treating a sentient being, a thing with intelligence, personality, desires and dreams, like it's just ... a thing. If she thinks she can just send it off to be "reeducated," if anyone's alignment needs to change, it's
hers.
I don't get this. Is changing a magic item a rule thing I haven't noticed? If it is, that's a thing I'd house-rule out of existence.
Attempts to change an item have no effect other than to unmake the item. It is what it is. The battle inside the pocket dimension would "neutralize" the weapon, all right - it'd remove the evil sentience entirely, along with all the benefits that that sentience brought. At most it'd end up a simple +1 sword. Further, if it's an artifact or unique item, there's nothing short of a capstone-level quest that'll unmake it.
Even if I decided to allow the sword to be re-aligned, the powers of the sword are not those of a "good" thing. "Good" heals, or burns with radiant damage, it doesn't rot with necrotic damage. You don't just swap those powers out; a
death knight doesn't change its mind about killing kittens and suddenly become a
deva. It's still a death knight. Once you convinced the entity that killing kittens is wrong, it would simply refuse to use those powers, possibly lamenting its past misdeeds so piteously that it refuses further violence. At that point, the sword becomes a burden not because there's an evil entity within, but because it's a useless lump of metal.
That means you'd have to re-spec the sword. You'd have to unmake it, remove the evil sentient entity - with all the moral and ethical problems that entails - and then either entrap (or convince) a good sentient entity to take up residence in the sword. That's creation of magic items completely out of the scope of player characters, in my opinion. All of which in turn means if you
can do something stupidly simple like send it away to the shop for a frame-off restoration, it'd come back a completely different car, I mean sword. All of that would also cost a king's ransom, because the Arcane Brotherhood is essentially creating a new legendary magic item to spec.
At my table, it'd be a hell of a lot more simple.
1. No, you can't send it away, submitting it to the Faerun equivalent of sending a teen to a camp so they can "pray the gay away." That is itself an evil act.
2. Even if you could just send it off like that, what you want isn't possible without destroying and rebuilding the item, which will take months and cost an actual, literal mint's worth of coin.
3. You can try to convince it to change its ways. Every time you try to convince the sword it should be less evil (or refuse to do anything the sword wants, like killing kittens) there will be Conflict resolved according to DMG 216.
4. Once you
do convince it to be not evil, it'll simply stop working. At all. If you try to
make it work, there will be Conflict resolved according to DMG 216. (Of course the PC shouldn't know that until she tries to use it in combat.)
In the meantime, evil folks should be trying to steal the sword your player is so "meh" about. It's a legendary item, fer crissake. You think the Zhents or Red Wizards or whatever are gonna just go, "Yeah, whatevs"? Hell no! They're going to get it for one of
their heroes. If I were you, OP, I'd have rivals perpetually trying to kill that character and all of her friends in order to steal it. To keep it in line with your current campaign, OP, there's an paladin of Tiamat somewhere who
needs that sword, and she's plotting to get it, along with the Red Wizards - who assume all magic, especially evil magic, should be theirs - and the Zhentarim, who know its value to both the paladin of Tiamat and the Red Wizards, and think they can negotiate a deal. Hell, throw the Harpers in as well, who want the sword to keep it out of evil hands entirely, either by destroying it or sticking it in a vault somewhere safe.
Taking possession of an evil, sentient artifact or item has to have consequences. Good characters don't take possession of an evil item unless they have a plan to destroy it or prevent evil NPCs from getting it. Yeah, it's tropy, but this is a game of tropes.
That's my take, anyway. I'm usually all about trying to find ways to give the player what she wants rather than ways to deny her, but that has limits. This is one of those limits.
Cheers,
Bob
www.r-p-davis.com