We have probably all experienced instant death attacks. The poison of a giant spider. The petrifying gaze of a medusa. The disintegration ray of a beholder. We fear these monsters (or at least we should) because of these special attacks. One moment our hero is exploring a dungeon and finding wealth, and the next they are gone. This is a different kind of horror than level-draining of undead monsters. A wight that drains a level of experience does not end your character (unless you happen to be 1st level). You can regain experience. You can keep moving forward to achieve your goals. But instant death attacks are the end. Or are they?
Many monsters possess this power to end lives. Poisonous centipedes, frogs, snakes, spiders, and more force a saving throw to avoid death. Cockatrices, gorgons, and medusas can turn you to stone unless you make your die roll. Beholders. Well, what can I say. But then there are the special kinds of instant death attacks that do not get a saving throw. A catoblepas or basilisk can kill you without any chance of saving yourself. You happen across their path and it might be time to roll a new character. I think these monsters are great additions to the game. They force players to think about not only “if” they should fight but “how” they should fight these monsters.
Not all death is permanent. There are spells (clerical and druidical) and magic items that can help mitigate death. Poisons can be slowed or neutralized altogether, as if the failed saving throw never happened. Players might be more brazen to attack a poisonous beast (or open a trapped chest) when they have such spells on hand, and those same players might be fearful of such things when the spells are gone. That is part of the game’s strategy. A character turned to stone can be restored to life by high-level magic. Even those characters that die have a chance of being raised, reincarnated, or resurrected. In fact, only a few monsters can outright eliminate a character from the game, such as the beholder’s disintegration ray or the deadly green slime.
A fair dungeon master will provide obvious signs that the more deadly monsters are in the area, giving players the option to proceed or not. Webs or old snake skins might suggest poisonous animals. Statues of humanoids with frantic expressions and positioned in odd stances would be a great portent of monsters with petrifying stares. The lack of life in an area might suggest that something large, sinister, and hungry hunts that portion of the dungeon. Broken doors, skeletons or fragments of them, ruined furniture, or horrible smells or sounds could also suggest something deadly occupies the halls ahead.
These have been my thoughts for developing the dwarven dungeon’s perils. Everything should always come down to the players. Yes, instant death attacks come down to a die roll (or not), and it can seem unfair that a hero you have developed for months is now gone. But that is part of the game, and I think the rules are justified. If every hero always lived, there would be no monsters, no dangers, no dungeons or dragons.