A New Calamity Table for Threshold-Limited Magic

Threshold-Limited Magic is one of the great innovations for the standard GURPS Magic system. It was introduced in Thaumatology, based on S. John Ross' Unlimited Mana article from way back when. Threshold-Limited Magic dramatically increases the size of a spell that a wizard can cast while giving wizards the option to risk their lives by casting too much magic too quickly.

Unfortunately, the risks of casting too much magic too quickly are too great. Wizards who use more mana than their Threshold have to roll on the Calamity table, with increasing risks of awful things happening the more the wizard is over the Threshold. Even for a wizard who barely exceeds the Threshold, there is a 25% chance of a fairly severe penalty (Threshold reduced by 33% or more, -3 on all spellcasting rolls) lasting for weeks, and a 5% chance of the wizard acquiring a permanent disadvantage. Greatly exceeding the Threshold and rolling poorly on the Calamity table can cause the wizard to lose the ability to cast spells, permanently alter the state of magic in the world, or possibly explode.

The reason that the severe results of the Calamity table are so unfortunate is that they act as a solid brake on wizards exceeding their Threshold. The Threshold/Calamity interaction is a neat mechanic, but with the results so severe, most wizards aren't going to risk making a Calamity check at all. I think even risk adverse wizards would be more likely to go over their Threshold in difficult situations if there was a bit more of a safety margin.

I mentioned rewriting the Calamity table for my College Ritual Book magic system, and here it is.

Revising the Calamity Table: Design Goals

Thaumatology explicitly states that the sample Calamity table is a suggestion, and the GM can adjust and expand it for flavor and style. What should be the design goals for a new Calamity table?

• Any game mechanic that is only used voluntarily needs to have a positive risk-reward ratio. Rolling on the Calamity table should not be such an obviously bad deal that no one will ever do it. If it is, there's no point in writing up the Calamity table.
• Bad results on the Calamity table should affect the wizard, not anyone else around him. It should not be possible to use the Calamity table as an offensive weapon (for instance, casting a huge spell and using the magical backlash to destroy mana-dependent creatures).
• Minor effects on the Calamity table should fade in a playable period of time.
• Increasing the amount the wizard went over the Threshold should increase the odds and severity of bad things happening.
• It should possible to achieve bad effects without exceeding the wizard's threshold by unreasonable amounts.
• It may be possible to get into some kind of death spiral, but it shouldn't happen just for slightly exceeding Threshold.

The overall principle is to encourage wizards to risk going over the Threshold, so as to actually use the Calamity mechanic.

Calamity Checks and the Calamity Table

To make a Calamity Check, roll 2d and add 1 for every full 5 points by which tally exceeds threshold after this casting. Calamities take effect immediately, but don’t normally cause the spell to fail.

Roll	Effect
2-8	Nothing happens -- this time.
9-10	Until the wizard's tally is reduced to 0, the base cost of casting that spell or ritual is doubled. Increase the multiple by 1 every time this result is rolled again.
11-12	As 9, and the wizard has a -1 penalty to cast that spell until his tally is reduced to 0.
13	As 9, and the wizard has a -1 penalty to cast all spells in the same college until his tally is reduced to 0.
14	Until the wizard's tally is reduced to 0, the base cost of casting all spells or rituals in the same college are doubled. Increase the multiple by 1 every time this result is rolled again. Also, the wizard has a -1 penalty to cast all spells in the same college until his tally is reduced to 0.
15	As 14, but the casting penalty is -2.
16	As 15, but the casting penalty is -3.
17	As 16, and the wizard's Magery is reduced by 1 (recalculate threshold and recovery rate based on the new Magery rate). This effective is cumulative. 1 level of Magery is recovered every day that the wizard has a tally of 0, doesn't cast any spells at all, and makes a HT check.
18	As 17, and as long as the wizard's Magery is reduced, he suffers from Nightmares with a control roll of 9 or less.
19	As 18, and as long as the wizard's Magery is reduced, he suffers from the -10% version of Radically Unstable Magery (Thaumatology p 26).
20	As 18, and as long as the wizard's Magery is reduced, he suffers from the -30% version of Radically Unstable Magery (Thaumatology p 26).
21	As 20, and the wizard is in a Twisted Mana (Thaumatology p 60) zone that only affects him for the next d6 hours.
22	As 20, and the wizard is in a Twisted Mana (Thaumatology p 60) zone that only affects him for the next d6 days.
23	As 20, and the wizard is in a Twisted Mana (Thaumatology p 60) zone with a radius equal to his base Magery (minimum 1) for the next d6 days.
24	As 20, and the wizard is in a Twisted Mana (Thaumatology p 60) zone with a radius equal to his current Magery (minimum 1) until is Magery completely recovers and for d6 days after that.
25-28	As 20, and the wizard is also a Mana Dampener with the same radius of affect as the Twisted Mana zone.
29+	The wizard takes (Calamity Modifier / 20)d of injury, reduced by half on a successful HT-6 check. If the wizard dies from this injury, their body explodes (reducing the wizard to -10xHP in the process) and doing the same damage as a cr ex centered on the wizard's hex. Should the wizard survive, also apply the effects of 25, above.

Design Notes

The default 3d Calamity table has a huge range: 3 to 18 on the unmodified dice. If barely exceeding the Threshold isn't supposed to do much harm, then nothing bad can happen until a 19+. If nothing bad happens until a 19+, either the severity of bad stuff happening has to increase very quickly, or the amount of going over the Threshold has to increase a lot. Reducing the table size to a 2d roll shortens the range to 2 to 12, meaning that bad stuff can start happening quicker.

Still, it's reasonably hard to reach the bad parts of the table. The first really unpleasant result is a 17, which requires exceeding Threshold by 25 and getting a really bad roll. Most wizards are going to stop there, especially in games where threshold is related to Magery and their effective threshold just decreased, but continued casting could get them into a death spiral of critical failures, low and twisted mana zones, and eventually personalized explosions. Magic should have some risk.

But with some lucky rolls, it's possible to have a tally that's 50 points over threshold and nothing worse happening than some penalties to individual spells or colleges. Most effects clear up in a week or three without casting spells.