Blinded

Blinded is a status effect which significantly lowers a creature's visual range. Depending on the game edition it causes:
* A -4 penalty to AC and THAC0
* A cumulative -10 penalty to THAC0
But the main interest is the effect on visual range, which plummets to 2 from the default of 14Source: https://gibberlings3.github.io/iesdp/opcodes/bgee.htm#op262. That a blinded creature can barely see anything means they can hardly target anything either, making them much easier prey. It is one of the status effects least resisted by enemies in the game. 
Blindness also prevents enemies from using vision-based attacks, for example the gaze attacks of Basilisks or a Beholder's eye rays.
Blindness can be inflicted by the spells Blindness, Prismatic Spray, Glitterdust, Sun Soulbeam, Holy Smite, Sol's Searing Orb, Sunray, Nature's Beauty, Power Word, Blind and Wild Surge #78. It can also be caused by the special attack of the Smoke Mephit.
Magic Resistance and Protection From Magic can prevent all the above effects. Spell Immunity can prevent Blindness and Nature's Beauty if cast with immunity: Illusion, Holy Smite if cast with immunity: Necromancy, Sunray if cast with immunity: Evocation, and Power Word Blind if cast with immunity: Conjuration. Potion of Magic Blocking can prevent the effects of Blindness, Holy Smite and Sunray.
If affected, one can cure the Blinded effect with Cure Disease, Neutralize Poison, Dispel Magic, Remove Magic, Heal and Protection From Magic. Potion of Magic Blocking can remove the Blinded effect of Blindness, Holy Smite and Sunray.