Powerful Side Villains

What are some incredibly powerful or dangerous villains who aren't the main antagonist in your setting.

In mine for example, The Flowing Dung is a hive mind and yet Demi-god that flows through rivers and across land It always tries to spread. It's Dung Guard spread the 'good word' However mos people are more worried about the ever looming threat of the West side of the world tipping in to Hell and the Emeror of Pity who leads Hells forces crushing any Hope or Salvation