Hi there! So I’ve been running a mainly home brewed campaign with five players for a long time now. This campaign has a big focus on character development and plot, and all of my players put a lot of time and effort into their characters. We’ve gone through countless big story arcs now, and all of their characters have received some major development. The Orc ranger turned to the dark side and multiclassed into a warlock, the ‘human’ wizard found out that he’s basically a genasi, the amnesiac skeleton cleric has been getting visions of her past, and the nocturnal (home brewed batfolk variant) pirate has finally paid his respects to his deceased crew. However, our Minotaur Paladin has not gotten as much development. For some background, Harley (the paladin) is a lawful good ray of sunshine who’s completely dedicated to spreading the holy light of her goddess. She’s also a member of the Inquisition, a powerful and widespread religious organization that’s played a major role in our campaign. Recently, Harley came across an enchanted, possibly cursed great sword. Despite the warnings of our astute cleric, Harley started using this sword religiously, wielding it even more than her standard inquisition greatsword. Harley’s player has repeatedly expressed interest in testing her character’s loyalty to her deity, so I’ve been considering having this cursed greatsword slowly impact Harley’s psychological state. If she continued to use it, it could slowly warp her. Eventually, Harley’s alignment could shift to lawful neutral, and perhaps even lawful evil. This is a very interesting premise because: A. Harley is the moral center of the group and the glue that holds them together. B. Harley’s goddess does not tolerate evil in any form, so this would likely cause some intense friction between the two of them. C. All of Harley’s major developments have occurred because of the input of other characters (finding out the skeleton cleric is undead, the Orc ranger’s corruption, building a relationship with the party’s chaotic neutral rogue, etc.)

I believe that this would be an exciting and fun idea, however, I’m not completely sure how to go about it. What would y’all suggest for this?

Well in Neverwinter Nights, there was a mechanism to do this. They rated your alignment on the Law-Chaos and Good-Evil axes on a scale of 1 to 100, I think. So 1 would be evil, 100 would be good. 1 would be chaos, 100 would be law. Neutral is 50. And sometimes, depending on the module maker, when you did something evil, you would lose "points" of alignment. This was called, in the NWN modding community, "taking an alignment hit." Take enough hits, and good becomes neutral, neutral becomes evil. Or do good deeds, and evil goes up by +1 or +2 to neutral, then to good.

Some people hated this mechanic, others loved it. But you could do something like this. Start her out at 100 Law, 100 Good. Every time she uses the sword (maybe every time she kills with it, or something), subtract 1 from the good/evil axis. When she gets down to some range around 50, make her neutral. I think in NWN, it was 75-100 = good, 1-25 = evil, 26-74 = neutral, but you can do it however you want.

I like BioWizard's suggestion as a way of tracking it. However, rather than just wielding the sword and changing a number, I'd require the character to do something to cause their transformation. Alignment should shift as a character's mentality shifts, rather than because you're being told that it has. If you don't act evil, you aren't evil no matter what sword you're holding.

Instead you might have the sword try to persuade Harley to commit unlawful or immoral acts. Perhaps the sword only functions at its full power if the wielder is committing dastardly deeds. Some of those deeds could even be enforced by the sword after the power has been used. Here's an example of a potentially evil weapon that provides RP opportunities.

Greatsword of Malicious Intent
Attack rolls and damage rolls made with this weapon are made with a +X bonus. (set the bonus level based on the party level and game tier)
The sword has three charges, which replenish at nightfall.
On making a successful attack roll, the player may expend a charge to deal Xd6 bonus damage. Using the charge also causes the weapon to ignore resistance to slashing damage.
Whenever a charge is expended, the wielder makes a DC15 Wisdom saving throw. On a successful save nothing happens. On a failed save, they gain one point of Malice. Keep track of the Malice rating. Malice cannot be reduced except by a Wish spell.
1-5 points of Malice: The owner is plagued by strange dreams.
6-10 points of Malice: The owner gets rid of all their other weapons and will not allow the Greatsword out of their sight. If anyone tries to remove it, they must attack that player until that player is dead, or returns it.
11-15 points of Malice: The owner feels a need to drink the blood of the living. If they do not drink 1 pint of blood from a living humanoid once per day, they suffer a point of Exhaustion at nightfall. The owner must make a DCDC15 Wisdom check each time they attack or must expend a charge. The owner's eyes turn red.
16-20 points of Malice: The owner needs to drink 2 pints of blood per day, and the blood must come from an innocent member of their own race or suffer a point of Exhaustion at nightfall. The owner's hair falls out.
21-25 points of Malice: The owner must drain an innocent of their own race of blood daily and consume the blood or suffer a point of Exhaustion. The owner can cast the Darkness spell as a Bonus Action three times per day.
26 or more points of Malice: The owner grows a pair of bat-like wings that give them a flying speed of 30 feet.

Screw my idea... do the Malice one. That thing is awesome. Use it and you essentially become a vampire.

I think the solution is actually a lot simpler. Talk about it with your player. Ultimately, she'll make the decision of if and when her character becomes corrupted, although you could definitely introduce things that lead her along a corrupted path. Maybe she starts having dreams that forewarn her, correctly, of traps and betrayals...then start warning her, falsely, about the other party members. Maybe she hears voices of departed friends in her head, cheering her on when she commits vicious acts. But ultimately, both the corruption and its ultimate outcome should be driven by the player: you can't force her to turn evil, even with a curse.