Okay, let's build a Big Bad for a DnD campaign, and think of approaches that will mean it's easier to run the Big Bad in your campaign, in a collection of neat ideas, which can be summarised as:
- Have a theme for your campaign
- Decide what your Big Bad poses a threat to
- Give your Big Bad a clear long-term goal
- Play them like a character
- Give them allies and enemies in the world (make them embedded)
- Let them have their moments
- Make sure there are ways that your players can slow the long-term goal without undoing any final fight.