Varsovian:
Interesting! Thank you very much.
I see that the "atmosphere of disaster and doom" is mentioned. Does it really appear in the book? Are there any rules related to it, like in Call of Cthulhu?
The Summon spell.
Level 1 Magic User spell, so accessible to any half baked mage you happen to meet.
In order to successfully inflict a serious doom upon the world, just get a level 1 Magic User with Summon to wander to the top of the highest mountain you can find, with lots of chickens, and start casting.
You have a 65% chance to fail your initial save and summon Something Stranger Than Expected.
You then have a 5% chance of getting an Abstract Form.
The Abstract Form has a 10% chance of being World Under Water, in which case the oceans rise to a height of 50 feet above your mountain top, at a rate of 10' per Turn. After 1 day, that's 1,440 feet of water, so anything not a mountain or hill is below the waves - and after a week, water is everywhere.
So in all, each time you cast the spell, there is a 0.325% chance of the world ending. That's more than the chance of rolling two 20s in a row (0.25%).
We now assume that the mountaintop Magic user has somehow failed to drown the world. Yet. Let's move on to the next worst case.
The Magic User is summoning something, and wants it to be as dangerous as possible. He is, however, hungry and cheap and will therefore eat all the chickens rather than sacrifice them.
Level 1 Magic User gives a +1, 2 hit die creature (maximum at level 1) gives +2. A Great Margin for the summoned Thing is 6, so a double Great is 12.
Thing rolls 20: auto-double
Thing rolls 19: 95% chance of auto-double, 5% chance of tie
Thing rolls 18: 35% chance of double, 5% chance of tie
Thing rolls 17: 30% chance of double, 5% chance of tie
...
In case of a tie, there is a reroll but the Magic User uses a d12, so the creature's odds go up.
Thing rolls 19 or 20: auto-double
Thing rolls 18: 58.33% chance of double
Thing rolls 17: 50% chance of double
...
So ignoring ties, the creature has a 16.75% chance of getting a double. In which case, it then has a 5% chance of opening a gate that spawns a thousand or so horrors. The magic User is up in the mountains, so it'll take a while for them to hit civilization, but it's a nice source of Doom too!
So between the two issues above (and you can wreck the local kingdom by casting Summon Monster and getting a flood, even if it won't then go on to drown the world), you have a better than 1% chance of ending the game each time the spell is cast.
Do you feel lucky?