This roll left me scratching my head… the table explicitly and specifically says not to roll on the trigger table for this trap, since it's a spell trap. OK… but a Blade Barrier isn't exactly a trap if it's just sitting there, right? And if the default duration, according to OSRIC, is 18 rounds, the party won't even encounter it if there's no trigger… unless you're a little extra creative. It's this kind of creativity, where you have to not just color, but color outside the lines a little, using tables for inspiration rather than for dictation, that I'm really thankful to the OSR in general for teaching me and for giving me a game where I can exercise it.
10: Blade Barrier
Description: In a room or hallway, there is a fireman's pole hanging from the ceiling and a hole in the floor that it descends into. A meter or so below the floor, the pole is greased to the degree that characters cannot slow their descent at all, and fall at the speed of standard free-fall. The pole leads to another level or sub-level, but the pole ends an appreciable distance above the floor of this level so that getting ahold of the pole to climb back up, even if the grease can be dealt with, should be nigh-on impossible. Cruel refs can make the distance between the beginning of the grease and the floor of the second level quite a ways; kind refs may make the greased portion of the pole short and interpose a chute between the end of the pole and the second level to mitigate falling damage. Falling damage should be assigned consistent with the system being used.
On the second level, players find themselves in the middle of a whirling circular wall of all manner of swords, the result of a Blade Barrier spell. Anything passing through the Blade Barrier takes 8d8 damage. Every 15 minutes, a Magic Mouth on the wall casts a Blade Barrier spell of slightly different radius from the one currently in effect. The Blade Barrier reaches up to the ceiling and down to the floor of the chamber the characters are trapped in. Every day or so, some malevolent denizen of the dungeon drops by, dispels the current Blade Barrier spells right after one is cast (so there is 15 minutes to deal with those trapped) and deals with the victims of the trap in a way consistent with their nature. Possibilities include eating, enslavement, robbing and leaving the party destitute, interrogation, etc.
Detection/Disarming: Any character who jumps down a hole, fireman's pole or not, without at least a cursory examination is just asking for it. Smart players' characters will examine the pole and hole before jumping in. One way to do this is to fly or levitate into the hole without the aide of the pole. Another, more common, way would be to throw or lower a torch down into the hole. There should be a 50% chance that a torch going into the hole (whether held by a character or not!) lights the grease on fire. That alone should be plenty of warning to the party! If the second level is close enough to the mouth of the hole to see, and if no chutes block the view, the party should be able to see an open floor, but not the Blade Barrier; the party should be told that they can see strange shimmering on the floor, though, from the torchlight reflecting off of the spinning, whirling blades.
Once trapped, characters will probably need magic to escape, becoming gaseous or incorporeal or dispelling the Blade Barrier. If the Magic Mouth is smashed, it will no longer cast Blade Barrier, so one wizard with a way to get out of the the Blade Barrier with a club or mace of some sort should make short work of this trap. Of course, getting out of this new level is another matter entirely…