Consequences as Conditioning
I’m a member of a Facebook group where a few friends who are all DMs compare notes – we pose ideas we’re working through (problems, plot holes, or campaign premises), and we help each other brainstorm solutions. One of my friends gave a player a magic shield, which is telepathically prompting the player to go seek out another magic item (a magic sword). She wasn’t sure whether the sword itself should be magical as well, or if it should be cursed.
I’ve thought a bit about how your behavior as a DM can condition your players, so here was my reply:
So, as far as whether the reunion should offer a bonus or curse, here's what I'd ask: how do you want to condition your players?
I don't necessarily think it's always the DM's job to "teach" the players, but I think the plot you establish WILL teach the players some sort of lesson (regardless if it's the one you intended or not).
If your players trust an NPC who betrays them when there are clear warning signs, it will encourage them to be more cautious when they are far too trusting. If your players are betrayed by an NPC who they trusted, it will likely teach them not to trust anyone, which will inhibit future interactions with NPCs.
To a certain degree, you can't even try to predict how your players will react, but the actions your characters take/the directions your plots take will inform them of the rules of this universe.
The same can be said for this side-quest. Do you want to warn them of the dangers of magic, and trusting magic items that have their own agendas? In that case, I'd recommend that the shield and sword have some penalty. If you want to make sure they don't both go to the same wielder, then perhaps being held by the same person will prompt that person to go into psychotic, uncontrollable rages, thus (hopefully) prompting the party to split up the items... and teaching the party to be smart about their magic items.
Even if the above is true and the items can't be trusted when held by the same person, you could still totally have them grant a bonus - perhaps when the two items are together/nearby, they grant a boost to saving throws (similar to a paladin's aura of protection). That might start messing with your Murder Math in unexpected ways, so I'd hesitate to implement that before you're certain it's what you want.
It also depends what level the party is – if they're 5th level and above and they have a spellcaster, they're never too far away from finding a "remove curse" spell, so having the items be cursed might only be a temporary inconvenience.
Lastly, the items could pave the way for another mission – who forged the item? What was their purpose? Perhaps they still feel there's a battle to be fought, and they encourage the player to hunt dragons or giants or goblins; this may not be a specific quest, but a nagging feeling in the back of their head that encourages behavior.
Alternately, the two items united could actually present a new plot - perhaps an illusion of their original wielder appears like Princess Leia and tells them about untold treasure (and perhaps tying it into the backstories of one or two OTHER players besides whoever has the shield). That way, you're rewarding the entire party for following one player's ambitions to get this shield, thus (hopefully) teaching the party that the rising tide rises all ships, and encouraging side-quests that help the party.
This is something I’ve thought about for a while; a couple of years ago, the same thread kept popping up on my Tumblr (this was clearly a while ago) about all the weird and bizarre things players do to derail campaigns. Seducing demons, that sort of thing. And a lot of these DMs seemed to take it personally, like the party members were doing these things for no reason at all, or doing it deliberately to screw with the DM’s plot.
I can’t speak for any of those other DMs or players, but from my own experience, if a player screws around and makes bad decisions, it’s because they don’t understand the potential consequences. Your job as a dungeon master isn’t to punish players for messing with you or your story – your job is to come up with a natural consequence for their behavior.
That being said, something to bear in mind as you plan your adventures is that your story dictates the rules of the world. It’s fun to introduce plot twists, but if your NPCs betray your players frequently enough, then your players will not like or trust anyone. We can see this play out in shows like Critical Role – in the first arc, the party fully trusts Clarota, a mind flayer with selfish intentions. In the fourth arc, they make a very tenuous alliance with a dragon. In the final arc of that campaign, Vax sneak attacks a monster that probably could have helped them because he didn’t trust the creature implicitly. Because, by this point, the party has learned that those alliances do not end well.
Let’s examine a film that shows this example: Casino Royale. In the course of that story (spoilers for the best James Bond movie ever), Bond is betrayed, and he believes the lovable René Mathis (Giancarlo Giannini) to be the snitch. The villainous Le Chiffre (Mads Mikkelsen) even confirms this when he takes Bond captive. However, at the end of the film he discovers he’s been betrayed by Vesper (Eva Green). It’s likely that Mathis was innocent (and this is even confirmed in the mostly-forgotten sequel Quantum of Solace) but at the end of the film Bond insists they should keep interrogating Mathis. In his eyes, Vesper’s death does not mean Mathis is necessarily innocent.
At the end of the film, James outright says he doesn’t trust anyone, and M replies, “Then you’ve learnt your lesson.” Because this is the premise of the film: not to trust anyone. The behavior of the characters reinforces the message of the world, which is not to trust. And if you want to tell a story of espionage and intrigue, then that makes perfect sense, and reinforcing those sorts of themes is appropriate. But if you don’t handle this sort of subject matter carefully, you can give your players a complex about trusting people... which is exactly what happened to the cast of Critical Role.
It’s possible you’ve also been setting your players up for this kind of behavior without realizing it. The next time you ask, “Why are my players always doing ____,” you should think back on your interactions with them and consider if you’ve been pushing them towards this behavior. For example, why do the players in my Sunday morning game always want me to draw a map for their fights? Well, thinking back, it’s because my attempts at theater-of-the-mind DMing combat hasn’t always been successful, and we end up moving tokens around anyway. I prefer theater-of-the-mind, but sometimes the players just need a visual reference, so we end up drawing makeshift maps. And that's partially because of one of my biggest DM weaknesses: white room syndrome. (More on this another time).
Why did the members of my first Curse of Strahd adventure dread traveling between Krezk and Vallaki? Because almost every time we passed through the crossroads, we wound up in an epic, intense combat (a few of us were killed at the very same crossroads). This fear caused us to start casting pass without trace on ourselves whenever we were within 30 minutes of the crossroads, just to be safe.
Why don’t your players take death seriously? Probably because they’ve been playing D&D for a while, and/or they’re high enough level that they don’t need to worry about it anymore – this was such a common problem in the D&D community that it became the hook for an entire published adventure, Tomb of Annihilation.
Players are not forces of random chaos, they are people who learn lessons from the world around them. If it seems like your players “aren’t learning anything,” consider why that might be – what sort of reinforcements are they getting? Even if something seems like an obviously bad idea, there must be something the player enjoys about it, or some justification in their mind.
For example, frequently in the first season of Critical Role, Grog made contact with some sort of sentient magic item (including two different examples of creepy talking swords). As a player, Travis enjoyed the insane bonuses the swords gave him, and felt they were far better than the very obvious drawbacks. Even though the first sword was clearly evil, that was fine because it made him powerful. But even if it hadn’t made him such a good warrior, I think Travis would have still enjoyed the experience, because everyone on that show loves drama and pathos, and Grog’s bizarre relationships with evil magic swords just continued to scratch that itch.
You have a choice when you establish your world – what are the rules? How silly is it? How dire are the consequences for splitting the party? Should deals with mysterious individuals be trusted, or are they always a bad idea? Consider the implicit lessons of your stories, and try to use those as a way to anticipate your players’ behavior.