The worst pain
Remember the worst pain that you have ever felt. It could be a cyst. Or breaking your arm. Or a shark attack. Anything that you remember as excruciating and something that you wouldn’t wish on your worst enemy.
But characters… they need some suffering sometimes. Write a scene where you take that pain and have a character experience it, but let them experience it from an outside source. This is not the character’s doing – if you broke your arm, let your character’s arm be broken. If you have terrible, awful headaches… let something be living there.
The antagonist is not, in any way, required to be human. I’m thinking monsters, you can think (and write) whatever you want, as long as the pain is coming from an outside source.