Simply put, a mod that has it so if the player is repeatedly damaged, small amount of damage or big the player will become in pain. During this vision could be blurred, the camera will shake, spooky sounds could play, tinnitus, the screen's corners could heavily tint black, nausea, and/or blindness could occur. The amount of damage, how quickly the damage is being dealt, and how long the damage lasts could determine which effects happen.
For example, quick and repeated dealings of small damage could cause small camera shaking. Slow, repeating small damage like starvation could trigger a mild nausea effect. While on the other hand taking lots of big damage like swimming in lava could trigger all of the effects. Burning to death could cause blurry vision and screen shaking, taking fall damage could trigger a very darkened screen and blurred vision. Etc.
There could also be some side additions like the screen brightening from black upon spawning. Or the screen fading to black upon dying.