Darkness

Darkness pulls in your field of vision. Torches, lanterns and the natural glow underground all shrink back, which is the exact reverse of what a Night Owl Potion does. Your character picks up a blue tint and closes their eyes for the duration, and that is the fastest way to tell the debuff apart from simply having wandered into an unlit cave.

It never kills you directly. The danger is where it shows up. Corrupt Slimes, Dark Mummies and the Brain of Cthulhu's Creepers all hand it out mid-fight, and losing sight of your arena for several seconds while something is winding up a charge is far worse than the hit that caused it. The Red Potion version lasts for hours, so drink that one somewhere safe.

A Blindfold blocks it outright. If you are already wearing an Ankh Shield or Reflective Shades you are covered without spending an accessory slot on it, which is how most players end up immune without ever thinking about the debuff.