Homebrew Need help with homebrewed Magic Surge mechanics

[Ranger Aarakocra]

Hey all, I'm working on a homebrewed D&D setting, and the main premise is that magic has been rendered unstable and sometimes does not even work. A formula for rolling after every spell casted is proving difficult to make. Here are the aspects that need to be a part of it:

• Level of the spell casted
• Magic instability rating of the area (1–8)
• Proximity to a leyline (channel of raw magic) in feet increments

Magic instability ratings for reference:

1: Magic just... doesn't work.
2: Magic is highly likely to fail.
3: While magic is more likely to fail than normal, you can still safely spellcast... for the most part.
4: Just the average amount of magical influence.
5: Spells are slightly more likely to surge here.
6: In an average combat encounter, you will definitely surge multiple times.
7: Spellcasting here is very likely to lead to a surge.
8: Cast Goodberry and grow a third eye, >70% chance of a level one spell surging.

Speaking of which, here's some other percentages for the likelihood of a spell surge:
In an area rated 4, there is a 5% chance of a level one spell surging. 5 = ~10%. 6 = ~24%. 7 = ~44% 8 = ~76% (these are probably unbalanced; this is just a rough estimate.)

Do you have any ideas for what kind of dice to use (d20, d100) and what the formula would look like? I'd appreciate any feedback.

 

[Ulorian - Agent of Chaos]

Do you anticipate any player rolling up a spellcaster for this campaign given these constraints? I know I wouldn't want to! Or is the idea that the party is pure martials? Or do you feel that spellcasting is too powerful and this is a mechanism to address that?

 

[aco175]

@Ranger Aarakocra welcome to he site, sounds like a cool idea but a heavy lift.

I like the spell-fail chance by level of spell. One issue is that you are adding another roll(s) every time someone uses an action. This tends to slow things down. If the payoff is worth it, then go for it. Not sure what the end point is though.

There should also be items, unless magic items also have problems, that allow you to drop the spell-fail percentage. The Wand of Potter takes 20% off so not you can always cast cantrips and level one spells unless you happen to be near the center of the meteor crash, then you are back to normal chance.

There also seems that the ley-lines would have grown towns and mage towers to help them cast without failure. Makes sense if this is the best area. Depending on how long ago magic became a problem, there might be all the dungeons along these lines as well.

This brings things to why the magic is having a problem. Is it just always been so and it is what it is. Did a meteor crash long ago and affects things the closer you are to pieces of it. Did a god curse things. Is there something the PCs can do?