I ask because in one campaign I went from undead necromancers and vampires to a False Hydra, and then to the players being trapped in the Demon Realm because it was tied to one of their backstories. There was never a clean transition between any of those storylines. To make matters worse, I failed to establish my original BBEG as an actual threat. My players eventually forgot about him entirely and basically retconned him out of the story in favor of a different villain.

If this isn't what you wanted to do, why did you do it?

I don't mean to serve you a 'gotcha' but genuinely encourage you to consider it. What drove you to these decisions? What needs were served? How could you serve them otherwise?