There are a few ways to create the ultimate evil for your DnD (or other media) game!
Choose from the prototypes of any popular mass media for the skeleton to build your villain off of!
The common tropes in media range from Evil boss to Mass-murdering maniac. The general tropes for a simple villain include a sad backstory, a tendency for murder, and a longing to take over the world by violent means.
The way to impart your generic villain’s actions is to really expand on the scope of their evil deeds. Don’t just mention the widespread destruction, but show it and its aftereffects. Have the village that your players were going to head to be torn apart, have the most trusted villagers mention their hatred of the Big Bad, and how the villain affected them and their livelihoods.
Examine your player characters and their motivations. Pick what drives them, and craft a villain that wants something similar, but does it in a completely different way than your player characters.
You can craft a foil from characters the player(s) already know, and they don’t even have to start as antagonists. With the foil, you can also grow their powers/abilities/fear-factor along with the players, to help with scaling their end-game “badness” level.
This is a villain built out of the world you have created. What would mess up the world you have created? What ideology would cause the collapse of the society you have? What is the worst case magical/supernatural/scientific scenario?
Say your world has a heavy dependency on a particular resource: the villain can hoard it, or they could seek to destroy all of it.