XUM20 Free-form Magic System

Expanding on my post about UltraMicrolite20 from a c0uple of days ago, perhaps the biggest shortfall of the system is the lack of a magic system.  There was the Open Ended Magic System (I think by Alex Schroeder, but not 100% certain of that), but I found it to be a little too crunchy for my tastes.  I’ve come up with an alternative, rooted somewhat in the Open Ended Magic System, that I think simplifies things a bit.

XUM 20 Free-form Magic System


In order to be a spellcaster a character must have the Knowledge skill as a Tag skill.

Limitations of Magic

1)      Magic can neither create nor restore life.

2)      Magic can change matter’s form, but cannot create or destroy matter.

3)      Like water, magic flows through the path of least resistance.

4)      The caster must be able to see, with his/her own eyes, the creature, object or area targeted by the spell effect.

Spell Points (SP)

Spellcasters have a number of Spell Points equal to their Mind score.  Spell Points are spent on spell effects, and regenerate over time.  Spellcasting monsters and NPCs have SP equal to ½ their Hit Dice, rounded down (minimum 1).

As an optional rule, casters can take hits to gain additional SP for spellcasting.  These hits can be healed magically, but otherwise require extended rest to recover.

Base Spell Effects

A spell is assumed to be an instantaneous effect cast on a single subject, object or area within the caster’s natural line-of-sight.  This line-of-sight cannot be extended through Clairvoyance or other means of remote viewing, though enhanced vision, such as infravision or x-ray vision, do apply.

Damage and healing spells have a base effect of 1 hit.  Spells providing bonuses to stats (STR, DEX, move rate, etc.) have a base +1 bonus.

Increased Effects

A caster spends SP to increase a spell’s base effect as follows:

Area of Effect = 1 SP per 10’ radius; spell will affect everyone in the AoE, friend and foe alike

Increased Bonus = 1 SP per +1 bonus to STR, DEX, Mind or move rate

Increased Damage = 1 SP per +1 hit inflicted

Increased Healing = 1 SP per +1 hit healed

Increased Spell DC = 1 SP per +1 to spell resistance DC or to spell attack roll

Meta Effect = SP cost modifier applied by the GM for more powerful effects

Stacking Effects = 1 SP per additional effect stack, plus any other SP costs associated with that effect

Wearing Armor = +1 SP per AC bonus of the armor worn by the caster

Sustained Spells

A caster may sustain a spell’s effects, causing it to stay in effect as long as the caster concentrates on sustaining it.  The spell ends immediately should the caster stop concentrating on it, become unconscious or die.  However, sustaining a spell effect makes it more difficult to cast later spells, increasing the cost of spellcasting by 1 SP per sustained spell.

Spell Attacks

Spells deployed as direct attacks that inflict hits require a Spell Attack roll:  d20 + Mind + Knowledge ≥ target’s AC.  For area effect attacks, make a single roll and compare the result to the AC of everyone in the AoE.  A roll of 1 on the d20 always misses.

Spell Resistance

Unwilling targets are allowed a spell resistance check to negate a spell’s effects on them.  Use an appropriate stat and skill rating for the check (use Hit Dice for monsters and NPCs).  The resistance DC is 10 + caster’s Mind + Knowledge rating (use 10 + Hit Dice + 3 for monsters/NPC spellcasters).   A d20 roll of 20 on a resistance check always succeeds.

GMs should assume that objects always resist spell effects.  Further, assume that any creature that does not, or cannot, consent to a spell resists it, even beneficial or benign spells.

Some examples:

Resisting paralysis, polymorph or poison = STR + Physical

Seeing through an illusion or invisibility = Mind + Subterfuge

Resisting charm or mental effects = Mind + Communication

Recovering Spell Points

Resting for a few minutes allows a spellcaster to recover 1 SP.  This means that casters effectively do not recover SP during combat, but can recover all their SP with a short rest after combat.

Example of Spellcasting

Mad Marko is a spellcaster with a Mind of 4 and a Knowledge of 6.  He’s wearing no armor and has 4 SP.  Exploring a dungeon with his friends, he decides to scout ahead, casting a spell on himself that makes him invisible and silent.  This costs 1 SP for the invisibility and +1 SP for the silence effect.  Furthermore, he’ll have to sustain the spell.

Spying a lone orc sentry, Marko quickly casts a damaging spell at it from down the passageway.  Knowing most orcs are weak, he only does the base 1 hit of damage.  Normally this would only cost 1 SP, but since he’s sustaining a spell, it will cost him 2 SP.  His spell attack roll is 9, + Mind (4) + Knowledge (6) for a total of 19, well over the orc’s AC of 15.

Marko’s now spent all his SP.  He’ll have to find a place to rest for several minutes if he wants to cast anymore spells.

After resting a bit and recovering his SP, Marko scouts out a room of orcs.  Feeling lucky, he decides to cut loose with a fireball.  Again, orcs are weak, so he goes for the base 1 hit of damage (1 SP), but adds a 10’ radius (+1 SP).  He’s still sustaining a spell as well (+1 SP), for a total cost of 3 SP.  Making his spell attack roll, he rolls a 1…disaster!  The spell misses and now he’s got a lot of pissed off orcs that know something’s up.

The GM makes a spell resistance check to see if the orcs notice Marko standing in room’s doorway.  The DC to notice him is 10 + Mind (4) + Knowledge (6) = 20.  The orcs get a 19 on the d20 roll, plus their 1 HD for a total of 20, just enough for one of the orcs to see through Marko’s invisibility and yell out a warning.

Marko beats feet back to his waiting friends with a pack of angry orcs hot on his heels.


While this system is a bit simpler to use, it is also considerably more powerful than traditional D&D spell systems or even the Open Ended Magic System, particularly spellcasting outside of combat.  Therefore, GM’s may want to place some additional constraints on casters, such as requiring 15/20 XP to increase skills (instead of the standard 10 XP).


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2 Responses to “XUM20 Free-form Magic System”

  1. thebabblingbane Says:

    I like it, though at initial blush, “Spell Attack roll: d20 + Mind + Knowledge ≥ target’s AC” seems a bit too powerful. This, I think, is evident in your examples where the Orcs need a 20 to detect him and the attack of +10 to hit the lone Orc. Assuming he is 1st level, this seems quite high compared to a conventional 1st level Caster in D&D.

    Can anyone cast? Can I add 1SP to attack something other than the target’s AC?

    • edowar Says:

      UltraMicrolite20 (and, hence XUM20) doesn’t use levels, but you’re right that a brand new XUM20 caster is more powerful than a typical level 1 D&D wizard.

      In the example above, though, Mad Marco is roughly the equivalent of a level 3 caster; in XUM20 a new caster would have a Knowledge skill rating of 4 and, assuming min/maxing, a Mind score of 4, for a total of +8.

      Also keep in mind, most attack spells in D&D just hit automatically, some allowing saving throws, some not. So, while a XUM20 caster can cast more damaging spells during the day, an attack roll is required for all of them.

      To be a caster a character must have the Knowledge skill as their Tag skill (from XUM20); character’s normally only get 1 Tag skill, so it’s a trade off with other skills (though the utility of magic does compensate somewhat).

      As written direct damage spells require a spell attack roll against a target’s AC. However, individual GMs may allow different types of damaging attack spells; indeed, they are encouraged to tweak the rules to their heart’s content. Casters can also get creative with their spell effects, say mentally dominating a target and causing them to walk off a cliff or into a burning building, for example (though an appropriate saving throw would be allowed, of course).

      Please let me know if you have any other questions. Cheers.

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