Puella Magi Arclights for adellhide as my part of the zexalauswap and I am SO LATE I’M SORRY!!!! I hope my interpretation of your prompt is okay!
It’s pretty feel-good, but it’s PMMM so of course there’s a tragedy waiting. Don K(yuubey) makes a contract with Byron, turning him into Tron so that he can take his revenge on Dr. Faker (as it goes in canon). Byron’s sons don’t know what happened to him—and unlike in canon, Tron doesn’t come to find them. Instead, they take it upon themselves to save their family: Chris is the first of the three to make a contract, wishing that “We can all be reunited as a family again.” Here, we have Chris discovering that his brothers have made the same contract. Don K promises them that if they keep fighting, their wish will be fulfilled.
This witch is one that keeps coming back: they can drive it away, but never defeat it. Eventually, the witch starts claiming them one by one—and comes back more powerful each time. In the end, it’s the last Arclight standing who realizes that the witch was their father all along, and that their wish to be reunited is, indeed, coming true—but not happily.