Seeing someone brutally torturing and killing everyone you care about, and then proceeding to do the same things to you. cutting open your stomach, slicing off appendages. Forcing you to stay awake by giving you drugs so you feel every moment of it. Things like that Basically the worst torture imaginable.
I don't know. There's a point where I think your mind would "break" and you would essentially shut down before it was all over.
Imagine being totally alert, healthy, and happy, with a fantastic life, and suddenly finding yourself 500 feet under water. It's excruciating because you struggle as hard as you can to reach the surface, which is so close you could taste that air in your lungs, only you never manage to reach it. Your entire body will start to burn for lack of oxygen, and start pumping adrenaline, but too late. After 5 minutes, or sooner, you black out or water gets into your lungs...and you KNOW it's over. Your exhausted oxygen deprived body can't even coordinate to move your arms and legs anymroe.
But you can just see the sun above the surface of the water. You can see the ripples and waves. And they get smaller, and smaller, and smaller.
Nobody will ever find your body in the black abyss of the ocean.