Last time I talked about how I dislike the design of dangerous monsters in oldschool DnD. The comments got interesting so I decided to talk a little more about how one could go about building deadly monsters effectively. The problem with old school medusas is not that they could turn people to stone - it is that the way you would go about avoiding that is to roll 1d20 to make a saving throw. That reduces what could be a fantastically memorable and dangerous encounter to a simpler number cruncher that occasionally kills characters outright. That seems like the worst of all worlds. On the other hand including monsters that have outrageous instant death attacks in the normal bestiary seems silly because what good are having stats for a beast that automatically kills everybody as soon as it gets a turn?
I think the answer is to have two separate bestiaries. The first one represents normal monsters that the characters can be expected to fight without notice or preparation. Gryphons might be pretty dangerous but you don't need a Lance of Gryphon Massacre to kill them, just good tactics and skill. Gryphons can have a normal entry with a number to indicate how difficult they are to defeat and regular combat stats. The second bestiary would contain monsters that are ludicrously dangerous to unprepared groups but require Plot to defeat. For example, a medusa could instantly turn anyone to stone who has not just anointed themselves with the water of the Fountain of Life. These entries can specify what is needed to be able to survive whatever hideous thing the monster does but of course GMs will often modify that to suit their campaigns. Including combat stats for a medusa is fine but the assumption there is that anybody who is needing those combat stats has already figured out how to overcome their stony gaze.
I have already included these sorts of things in the game via Epic Rituals. There are plenty of hideous and awful things an enemy could do right in the manual that the players would have to try to stop just as the heroes do in stories. By including epic monsters with extra text describing a default way to prepare for those monsters it becomes much easier for a novice GM to build a final boss that is a truly legendary encounter right out of the box. Of course I would make the second bestiary much smaller than the first - a dozen entries at most since eventually people will end up making their own hideous and unfair abominations. In theory they are there to provide ideas and inspiration rather than an enemy of the week.
Making encounters that are really terrifying and require lots of prep is a very useful thing to do. It definitely can amp up the reward of beating a particularly difficult opponent when much preparation was required to even step into the ring with them. The best way to accomplish this though is not to simply have cockatrices, harpies, and basilisks wandering the forest randomly but to use them as legendary terrors that must be approached with utmost caution and to make the rules actually support that.