The eye killer is a strange creature found in the first edition Fiend Folio. It looks like a bat/snake hybrid and can concentrate and reflect light to create a death ray. I guess the idea was to have demihumans, at least those with infravision, find the critter in the dark and kill it before the humans and their torches let the monster blast the whole party.

I don’t like the death ray, but to weaken it substantially would alter the nature of the creature as well. It is a good encounter in the style of AD&D traps rather than monsters. So I came up with a very easy way to allow the monster to keep its death ray and scare the crap out of the players and characters. The beam still does 3d6 points of damage and those who fail their save do not die, but rather lose the points permanently. The beam seared off some flesh, leaving a horrendous scar, lost fingers or the like (DM’s call). The only way to heal this damage is with a Regenerate or Limited Wish spell or a quest to a healing fountain.

I can’t find any other monsters with similar ray attacks, but it wouldn’t be difficult to apply the permanent loss to other death effects. They cause heart attacks, selective tissue aging or flesh to blacken or wilt. Whatever the effect, the points are gone until the character has access to powerful healing magic. This keeps most characters alive (sorry magic-user) and gives them a reason to either make enough cash or goods to pay for the spell or adventure for the fountain. And I think that makes using such a monster a viable option in appropriate leveled games.