Suggested by JMT
Camp Half Blood (The Percy Jackson universe) wage war against Hogwarts School of Witchcraft and Wizardry (Harry Potter).
Camp Half Blood’s composite Greek demigod forces, disregarding allies and gods, but including deceased or traitorous demigods, have reason to believe that one of their own is in Hogwarts, and has laid siege to it, using divine magic to block apparition off the grounds and floo travel. Hogwarts is in Sixth Year, possessing a full-power Dumbledore.
Can the Camp take the castle, how long does it take, and what losses are there on each side?