As summer comes to an end, two expensive fantasy series filled with sorcery, sword fights and fantastical beasts will premiere on rival streaming services. While it may seem like Amazon Prime Video’s “The Rings of Power” and Warner Bros. Discovery’s “House of the Dragon” should be dueling franchises, as they begin within a couple weeks of each other, the two series serve very different purposes for their respective studios. The stakes may be higher for “House of the Dragon,” which will go first. It starts Sunday on HBO and streaming service HBO Max, arriving as newly minted CEO David Zaslav...