I created the following short rules to model an earthquake created by the closure of a portal to the land of the dead. The narrative of how it played out is at the beginning of the Session #82 entry over at Empty Z.
The setup is a low rumble followed by shaking. It’s not an actual earthquake, rather a magic event that spans a few minutes. As a DM, I want uncertainty for myself to keep the action surprising. I also want the rules set ahead of time lest I feel tempted to improvise something overly influenced by my mood at that particular moment. That is, I want to be Crom, merely setting the world in motion.
The players are inside a church. The doors are spiked closed from outside. They can try to force the doors open using the rules for smashing open a locked door, which is rolling d10 and getting in the range of 1 plus their strength bonus. The strongest character has a 30% chance each round to make this happen. There’s also a way out they way they came: up stairs to the roof, across the roof, down a rope 30′ to the ground. They can take two rounds to safely repel or try going down in one round, but risk a save versus death or take 1d6 in damage.
I did not anticipate escape via the stained glass windows which I’d described a couple of sessions previously. I improvised that they were 10′ up and would take 10 damage before breaking apart.
The progression of the magical earthquake used the following procedure.
Each round, roll a die. Start with d12. Use a smaller die each round using the sequence d12, d10, d8, d6, d4 and then keep going with d4. Check the result.
- On a 1, the entire structure collapses. Anyone still inside takes 3d6 damage. Anyone on the roof takes 2d6 damage.
- On a 2 or 3, part of the roof gives way, either dropping a large chunk or opening a hole underneath one character. Choose the character at random.
The sequence of ever-smaller dice means collapse becomes more likely with each round. However, I can’t know exactly when, though most likely it would happen in five rounds or sooner. I communicated urgency. The players picked up on the uncertainty of what could happen and responded with creative problem solved that was both action-oriented and not combat.