Characters feast after one of their successful adventures. They eat well and drink hard. Next morning the stallion of the party discovers himself from a decorous bed of Regna, a handsome middle-aged woman. Unknown to the PC Regna was married to a mighty man, the archmagus of the Wizards’ Council. Now she is a widower. Well, PC can go on with his adventurous life without this small episode having any influence, until…
A year later, when the character has already forgotten about the whole thing, he receives a letter. Regna wants the PC to arrive at her manor as quickly as possible. Arriving there he finds out that she has had a daughter, Jassa—and there’s no question about father’s identity. As if this wasn’t enough to scare the adventurer, Regna tells that she is dying soon and has to leave Jassa to the father. The widower makes the PC vow that he will take good care of their child. Next night she passes away. (To make the PC feel guiltier, make Regna’s fatal ailment to be caused by a difficult childbirth.)
If the character didn’t give a vow or doesn’t take care of the infant, he will get colleagues of Regna’s late husband after him. A bunch of grey mages will make clear that the father has no reasonable option but to take Jassa: the wizards will warn him that they will not approve of any irresponsible behaviour. They may or may not tell PC that their loyalty to Regna is driven by the fact that she was the late archmagus’ wife.
How is the PC able to handle stressing life of an adventurous single dad? Does his party help him with the task or shall they try to find a way to get the baby into a proper home? The wizards will every now and then check that the girl is faring well. If she isn’t, they shall try to take necessary action to correct the situation.
To make the task of dragging a child around with the party easier, it is possible to decide that Regna was somehow magical herself and her daughter e.g. grows faster or has some ability which makes it easier for her to survive the most basic hazards of adventurous life (hunger, long travels and cold nights by the campfire). Or maybe Regna wasn’t a human to begin with and children of her race are inherently somehow more competent than human children. One way to make things easier for the party would be to make Regna give them an automaton or some sort of an item which would take care of the baby’s most pressing needs (e.g. The Diaper of Instant Purity). I would choose the automaton/item option rather than make Jassa something else than just an ordinary wailing packet.
Wizards are not interested in the child’s welfare only for their benevolence. After death of the former archmagus (Regna’s husband) the Council of Wizards was taken over by a usurper, Medharr, who is using the Council only for his own benefit. The usurper is also looking for Jassa in order to kill her. If the father proves to be reliable, the wizards will tell him about the whole thing. None of the wizards is ready to take the burden of raising the girl and it would be dangerous to give her to an orphanage, where she would be vulnerable. Child has to be kept safe until she is old enough to start learning magic. Wizards are hoping to make her the new archmaga when she reaches maturity - she is after all the only legal heir of the archmagus’ splendid mansion and vast magical library. Jassa is the only one with even a bit of old archmagus’ authority behind her.
You can make the party more involved in the usurper-stuff by giving them an opportunity to help Medharr - of course before they know anything about the girl. Medharr could be a kind old man - a basic wizard type. He gives the party a mission to get the platinum amulet of blah and the golden sceptre of mumble from the bastion of mumble-blah. The bastion is a small magical fortress built to safeguard the emblems of power normally carried by the archmagus - characters of course do not know this. After receiving the amulet and the sceptre old man rewards the party and goes away. The party hears nothing of him after that and go on with their lives. Meanwhile Medharr becomes a highly controversial archmagus. His position will not be approved by many, but anyway, there is no-one to unite the remains of the council to challenge him. When wizards tell the party about the Council’s crisis, they may also mention that the usurper got his position with assistance of some unknown rogues, who stole the amulet and the sceptre from the Council’s magical fortress. This should ring bells in characters’ heads and—if they are decent folk—make them feel a bit guilty.
I see three main ways to end the affair:
1) Characters leave Jassa or are unable to defend her against Medharr. In the former option wizards will probably try to take revenge. In the latter option characters may still go on fighting against the usurper.
2) Characters defeat Medharr. If this happens, one of the wizards offers to take care of the child now, when things are not so dangerous anymore. Don’t make this an easy option. Medharr should be strong enough to cause substantial damage to the party or at least escape if things get too rough. Most of the time he also has body-guard wizards around him. It is also worth noting that wizards opposing Medharr are not willing to attack him directly. No-one wants to see the catastrophe of a wizards’ civil war.
3) Characters take care of Jassa - probably for years - until one of the wizards takes her as an apprentice. If the girl survives apprenticehood, she may challenge the usurper. If she wins, characters will have a powerful ally and - of course - a daughter. Naturally it takes a lot of in-game time to get to this point.
Additional things to think about:
What are the responsibilities of the Wizards’ Council in your game-world? What are the consequences of division of that agency? Do things get weird? Who will benefit from the confusion and who will suffer? Does the situation force any extraneous societal or cosmic instances to enter the conflict?
There are many possibilities for connecting other adventures to this one. Despite the long baking time this plot doesn’t need to be a campaign. It can be just one vein running behind other adventures and plots. By playing the Jassa-does-not-grow-like-an-ordinary-child card it is possible to even run the whole thing in one session. Also by guiding the party to face Medharr early the thing could be over in a blink. But of course it could also take the time of a long campaign to reach the end.