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I want to design a campaign with a theme on dangerous magic, but I'm not sure how to go about doing that. I can't find any official rules for making the casting of spells hazardous in some way. Does such a thing exist, and where can I find it? If there aren't official rules, I'm also open to suggestions on house-ruling such a system.

What I'm looking for is a system designed around unintended targets, backfires, and unexpected additional effects. I'd like to avoid a skill check system if possible, and don't really like the idea of sanity/taint.

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House rule answers should adhere to the rules outlined in Good Subjective, Bad Subjective, be based on actual experience, and contain references and examples whenever possible. –  BESW Apr 15 at 2:14
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Please take a look at the answers to this question and clarify in what way the answers you're looking for need to be different. (I'm not claiming it's a duplicate, but they share enough superficial overlap that looking at it may help you provide more detail on what you want.) –  BESW Apr 15 at 2:18
    
I'm not an expert when it comes to 3.5e, but someone else may know whether there's an equivalent for 2e's Wild Magic. –  Dakeyras Apr 15 at 2:22
    
@Dakeyras There was a 3.X supplement that included a wild magic spell caster class, but it's been so long since I read it that I've forgotten where it was, or which variant of 3.X is was made for. –  GMJoe Apr 15 at 3:43

4 Answers 4

up vote 9 down vote accepted

What I'm looking for is a system designed around unintended targets, backfires, and unexpected additional effects.

This brings automatically to mind the Dead and Wild magic areas in the Forgotten Realms Campaign setting.

The main idea, is that all magic worldwide is woven like a weave and attached to the goddess of magic in the setting (Mystra). Some holes, (whole areas or so small like a 5' spaces) exist in the weave, and there any spell cast can have unpredicted effects. If the magic in the area is totally obsolete, these are the dead magic areas. No casting is allowed here, neither it is possible to cast arcane magic at all.

Wild magic areas are a little more interesting, as the casting of any spell will either backfire, non be cast at all, cast normally, thrown on a random target, or be miscast in other ways, (funny, interesting but also dangerous ways). You actually roll a d% to choose from a table with possible outcomes for casting in such areas.

You will find extended rules and references in both the Forgotten Realms campaign setting book and also the supplement book Magic of Faerun. But beware, both of the books are 3.0 edition and they will need some minimum conversion before becoming totally playable and not gamebreaking in 3.5 edition. Look for the book Player's Guide to Faerun, as not only it is a useful book, but it's the only supplement written in 3.5 and it is used to convert characters, spells, feats etc from 3.0 to 3.5.

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+1 I should've thought of that as well. :) –  OpaCitiZen Apr 15 at 9:24

Do take a look at the magic system implemented in D20 Call of Cthulhu (an official WotC product.) It's not exactly 3.5, but is practically 95% compatible with that iteration of the system - especially if it's only example and inspiration you're looking for. (But you could lift it in its entirety into 3.5 with very little effort.)

Spells in D20 CoC aren't tied to classes: they can be learned and used by anyone - for a price, paid in Sanity, ability score drain, etc. Spells can be also rather unstable, at the DM's whim.

Mind you, there are just basic rules and outlines system-wise: complications are mostly up to the DM/Keeper - but the guidelines and the whole write-up of the matter are pretty excellent. IMO definitely a good basis to develop your own "risky magic" house-rules.

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Yes, Wild Magic Planes are in the SRD.

Simply make the prime a "wild magic plane." (Though I'd alter the DC by area, just to not completely destroy spellcasters.)

On a plane with the wild magic trait spells and spell-like abilities function in radically different and sometimes dangerous ways. Any spell or spell-like ability used on a wild magic plane has a chance to go awry. The caster must make a level check (DC 15 + the level of the spell or effect) for the magic to function normally. For spell-like abilities, use the level or HD of the creature employing the ability for the caster level check and the level of the spell-like ability to set the DC for the caster level check. Failure on this check means that something strange happens; roll d% and consult the following table.

I'd remove the chaotic requirement of the wild mage PrC (complete arcane) and allow it to apply to arcane or divine casters. (Making only arcane, psi, or divine wild will strongly discourage use of that sort of magic.)

I'd recommend, however, using a rod of wonder as inspiration for one power source, and that planar chart for the other, interpreting as necessary. Be prepared for a single botched spell to TPK your party, though.

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Random magic : one of the first thing I wanted to try when I started D&D. Since I fumble a hit on two, my allies doesn't allow me to try this... :/

In the RAW

Like a lot before (and after) me will say : in the Forgotten Realm, the Wild Magic. If you want to add some particular place of instability, you should take a look at "Magic of Faerun", where some "Places of power" are define like Mystic Maelstrom , a black hole of magic, or a Spark, a node that augments magical effect

(Not really unstable but alternate and potentially dangerous kind of magic)

A friend tell me about a kind of magic based on the Constitution. When the weave disappear on some area, the magical user still can use magic but the energy they use to do it is their own vital strength instead of the Weave power.

The caster do a Fortitude saving throw or SpellCraft test (DC = 10+spell level). If he fail, he have Constitution damage (spell level/2). If the saving throw is successful, the caster only take half ability damage (spell level/4).

Note that this game mechanic is really dangerous when you don't know what is going on and the casters should become really weak rapidly since they use their life as ammo. Therefore, if you use that, remember that the enemies should be affect too and can be less "Good" than the heroes : I should propose a Lich who capture humans to cast spell with their vital strength but I'm sure you can imagine a lot of different things with that idea.

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