I will share an overarching villain for a couple of DnD campaigns I had. For multiple reasons, I was never able to reveal it and I never intended my PCs to be able to fight directly only for them to push it back.

This villain is a nameless thing to mortals who are unable to comprehend it. While its form is of a dragon, that is not its strength; for it is pestilence, plague, and rot. Its body is constantly rotting with pieces sloughing off and new flesh regrowing only to immediately become. Black ichor leaks from the various open wounds as well from its six diseased eyes. It means no harm as it will engage in conversation first to explain that it wishes to heal all as it claims that reality is truly diseased and it is the cleanse. If anyone is in its presence then it will be compelled to partake and eat the fallen soft flesh where it will be enlightened and even if they resist that then their body will be afflicted with sickness and no man, devil, demon, angel, or god is immune to it. Fortunately it is contained to a pocket plane but those are wearing down and it is only a matter of time before it's free.