Conceptually, this idea springs from the dungeon as not just a series of monster-infested tunnels, but a metaphysical entity, a locus of maximum chaos and magic. The reason so many evil wizards end up haunting dungeons is because that's where they are more powerful. The players can draw on that same reservoir of chaotic energy, but it has its risks.
So start your bag of beads with 3d6 good beads, 1d6 blanks, and 1-3 bad beads. Bad beads are seeded this way, roll d6 each session:
1-3 add one yellow bead
4-5 add one yellow and one orange bead
6 add one yellow, one orange bead and one red bead
I'll get to the three color of bad beads in a bit. The basic deal is that a good bead is one spell level. If you get one or more blanks you have to choose immediately: the spell is delayed at least one round or you can cancel the spell right now. However, if you've already fumbled (drawn the yellow, orange or red bead), that still takes effect. If you go ahead, what you have to do is draw replacement beads on your go the next round, unless the whole spell is cancelled because you take damage in between. If you get blanks again, you are in the same boat as on the first round.
Now here's where it gets interesting. You've got some spells memorized, right? Draw from the bag and you don't forget the spell. Or you can pay a premium to cast something else:
Spell memorized, but you already burned it today: Spell level +1 bead
You know the spell and can cast it, but didn't memorize it today: Spell level +2 beads
The spell is appropriate to your class and level, but not in your spellbook: Spell level +3 beads
The spell is appropriate to your class, but you are not high enough level to use it: Spell level +4 beads
The spell is not for your class: Spell level +5 beads.
That's right, under this system any caster can attempt to cast any spell in the rulebook. The real question is whether or not they are willing to chance a disaster. Disasters come in three flavors, mild (yellow), medium (orange), and extra spicy (red). And since a mishap represents the strange essences of the dungeon biting you back, sometimes they change the environment.
Sample Yellow Mishaps
- The spell gets away from the caster. No effect and caster cannot use that same spell for 1d6 days.
- Smoke comes out of the caster's ears. Take d6 damage, save versus magic negates, and -2 on all die rolls until you get a good night's rest in a proper bed. No save on the die penalty.
- Spell goes off as planned but caster gains some minor mutation of the DM's choice that signals their taint. Eg. red glowy eyes, a forked tongue, scaly skin, that sort of thing.
- The dungeon fights back with more darkness. As long as the party remains on this level non-magical light sources wink out at the most inconvenient times, never lasting more than 50% of the expected duration.
- Dungeon denizens become more attuned to your presence in some unconscious way. Double wandering monster chances as long as the party remains on this level and the caster remains alive.
- All spikes pounded in by the party, ropes left behind, marks left for navigation, etc., disappear, swallowed up by the dungeon itself.
Sample Orange Mishaps
- The spell gets away from the caster. Randomly determine target among all present and caster cannot use that same spell for 1d6 weeks.
- Electric energies coruscate up and down the caster's body. Take 2d6 damage, save for half, and any activity but walking at half pace is impossible until you get a week's proper rest.
- Spell goes off as planned but caster falls to the ground in agony for d6 rounds. When they recover, the caster has gained a random mutation. (No. I'm not going to give you a mutation chart. If you're a DM you should already have one or at least know how to google that kind of shit.)
- The walls on the dungeon start dripping blood. Climbing walls and getting purchase with a thrown grappling hook become impossible. Fumble chances are doubled. The monsters are unaffected.
- 1d6 extraplanar entities appropriate to the level (level 1 = manes, level 2 = lemures, etc.) appear in a random room or chamber within 40' to 240' of the party's current location.
- 1d6 dungeon features (doors, stairways, statutes, whole rooms) on this level or other levels visited by the party this session permanently shift to somewhere else on the same level.
Sample Red Mishaps
- The spell gets away from the caster completely and materializes as a hostile Spell Elemental. Use Fire Elemental stats (randomly determine size) and it can use its own spell effect to its advantage (e.g. a Magic Missile Elemental would be shooting missiles like a champ). Also, caster cannot use that spell ever again.
- Unknown energies burst out of every orifice of the casters body. Take 3d6 damage and everyone within 30' also takes 3d6, but they can save for half. Caster and everyone who fails to save is comatose for 1d6 weeks.
- Spell goes off as planned but caster falls to the ground in agony for d6 rounds. When they recover, the caster has gained a random mutation. Roll twice on whatever table you use and give the caster the more inconvenient, weirder, and/or crappier of the two.
- Cracks open in the walls. Giant clawed hands reach out and grab 1d4 random party members, dragging them into the body of the dungeon itself. The caster and any NPCs get no save against this effect, but other PCs may save versus paralyzation to avoid. The cracks close up and the lost party members reappear d6 weeks later, horribly changed. They are now monsters and implacably hostile to their old comrades. They will positively chase down the caster if they are not among the altered.
- Any being in the dungeon capable of scrying or other extradimensional insight immediately latches onto the party and will be able to monitor their activity any time any of them they are in the dungeon.
- The whole level dissolves into nothingness. All present on the level save versus spells. Failure indicates disintegration. Success indicates you are dumped in a random location above or below the level. Over the next d6 days the dungeon heals itself by forming new connections to reroute around the now non-existent level.
One final note: as you descend deeper into the underworld, more chaos energy becomes available to spellcasters. Add 1d6 additional good beads and one more blank every time the party goes down a level. Also replace the yellow or orange bead if it has already been drawn. One red bead showing up in a session is quite enough, I think.
So here's my question: You are a magic-user deep in the dungeon and nearly out of spells. The present encounter is going badly. The bag appears to have about a dozen beads in it. That could be 11 good beads, one blank, and no bad beads. Or it could be three good beads, six blanks, and all three bad beads. How desperate do you need to be to find out?