Summary
Sometimes, we end a campaign and intend to start another one in the same setting, and because of your players, there’s a bunch of content that will influence the future. Sometimes, a player sadly needs to leave the campaign, leaving a character behind that is now under your control. Whatever the case, it’s common for player characters to become NPCs inDungeons & Dragons.
They can become allies, mentors, or even villains, depending on who the character is and what they’ll be up to when your next campaign begins. We can’t tell you exactly what to do since we don’t know the character or the story, but we can help you figure something out.

5Continue From Where Their Story Ended
What Is Their Happily Ever After?
After having the barbarian PC go through their whole journey to save their clan from destruction by defeating their villain and finally return triumphant as their hero, wouldn’t it be nice to see them lead the clan?
If things went as planned in your campaign, these characters have achieved their goals. So, based on their personality and other ambitions, what would they do once the adventure is over? Will they be adventurers for the rest of their lives, dying in battle to save others? Will they become political or religious leaders? Will they just retire and openthe classic magic item shop?

It’s important to consider how much time will pass between your campaigns and how age and events in between will affect the character’s decisions.
you may deduce that from how their player behaved during sessions or if the character has ever said something about their retirement plan - or you can ask the player. Either way, that’s the perfect way to understand the new NPC’s starting point in the new story.

4Consider How They’d React To The New Story
Or How The Themes From Their Story Fits The New Plot
The barbarian, now the leader of their clan, sees a new threat arising, one that is a danger to the whole world and, thus, their clan. What are they going to do about it?
Considering our barbarian example, feel free to threaten their clan again and all, but we don’t recommend undoing their victories from the previous campaign during this time between stories - in our example, that would be destroying the clan during the time jump. Just undoing the player’s success out of nowhere is extremely underwhelming for them to see.

With the new bad guy and the dangers you’re planning to throw at your new party, how would the old characters react to it? In fact, would they even be aware of it? Because if not, adding them here might feel forced.
Still, after you establish what they’re currently doing with their lives, you may analyze if they would be aware of these issues and whether they would do something about them. The ideal scenario is to find an excuse for them to not handle the problem directly but perhaps give assistance, such as information, to the new group - it’s their story, after all. Or they solve one part of the issue while the group solves the other.

A fun way to mix things here is to make thematic matches rather than the plot. For example, imagine a former PC who had a revenge story that went poorly, and one of the new PCs has a revenge story, too. This former PC would be the perfect character to advise about not losing yourself to hatred and obsession, and they can say that from experience. Do the stories your current party is going through have any similarities with the stories the old gang went through?
3Consider Their Alignment
Are They Friends Or Foes?
Evil people may also save the world- it’s where they live, after all. Intentions are just as meaningful as choices, and now that these characters may have achieved powerful positions in the world or became literally powerful, their alignment is a very important detail to consider.
If the character is good, they would likely become an ally or maybe a mentor to the new party - they could even be the quest giver, telling the group to do something while they do something else, all to save people.
Neutral characters can just go live their lives and not care about intervening in whatever is going on, while evil characters could potentially be the cause of whatever problem is happening in your world. A former PC becoming the new BBEG is a wild thing to see and a fun one, especially for the player.
You could potentially have these alignments swapped in between campaigns, such as having a former good character be the new villain or a former evil character be a hero, but unless that’s a topic that was addressed during the previous story, we don’t recommend it. It could feel forced, and the player who made the character might not enjoy their hero becoming a villain out of nowhere.
2Give Them A Cool Moment, But Don’t Give Them The Spotlight Indefinitely
They’re Not The Protagonists Anymore, After All
NPCs deserve their time to shine, too, especially when it’s a beloved character for all the players involved. They can help in a fight, save the group, or something similar - adding them in a combat encounter is perfect for that.
We know they’re not the protagonists, but sometimes, secondary characters get their moments, too. Just make sure that not everything revolves around them, and the spotlight will return to the players after it’s all done.
If the story does revolve around this former PC, ensure they need the party’s help anyway and that whatever is going on is something they can’t handle alone. Ireena from Curse of Strahd is a good reference for that.
1Ask For The Former Player’s Feedback
Not Everything Needs To Be A Big Surprise
Like a director talking to an actor who played the same character for a long time, you can just ask the actor about what they think is appropriate for the character moving forward. We know that keeping mysteries is fun, but having the player’s input helps a lot. And you can still make the cameo a surprise for the rest of the party.
You can even let this player use their former characterin the backstoryfor their new character. It’ll make things easier to have this character reappear without feeling forced.
Ask them what they want for their character after the adventure is over, and start things from there. Sure, you don’t need to tell the player how the new story will affect their old character, but you can use their feedback to kickstart things.