Damage
ArM uses bonuses from weapons and other effects as one component when determining damage to a target. To convert these damage bonuses to d20 damage, use the following simple formula to calculate how many dice of d6 damage to do [Note 13].
damage dice = ArM damage mod ÷ 5
As an alternative, if you want to emulate the non-random nature of ArM (damage dice are not rolled in ArM), use the following formula instead.
damage = damage dice × 3
or
damage = ArM damage mod × 2/3
Example: According to the the spell description for Pilum of Fire (CrIg 43), on p. 135 of ArM, it does +25 damage on a successful targeting roll. In the d20 version of the spell, it would do 4d6+3 points of damage. [This would average about 17 points of damage.]
A spell or other effect that causes the loss of one Body level should convert to 3d6 of damage.
One playtester (thanks, GigaWatt Warlock) uses a simple equation to calculate damage done by any spell. Take spell magnitude (casting roll ÷ 10) as the number of d6 of damage to roll.
Stat Rolls and Saves
When converting ArM rolls to saving throws use the following guidelines (also, keep in mind the excellent guidelines from the DMG on saves vs. stat checks, p. 94). These should almost certainly be converted into saves when used as part of avoiding damage or effect of a spell.
This roll in ArM… |
…Converts to this in d20 |
|
Stamina check |
Fort save |
|
Int check |
Will save (may base on Int) |
|
Perception check |
Wis check |
| Quickness - Encumbrance check |
Ref save |
Fatigue
Fatigue is an instrumental part of spellcasting in ArM, and thus deserves special attention. To handle it, we introduce the concept of fatigue subdual damage.
 | Anything that would cause a level of fatigue in ArM will translate into 1d6 of subdual damage per magnitude level of spell attempted. See Note 11. |
| When a character has accumulated subdual damage that amounts to 1/3 of their current hit point total, they become fatigued. |
| When subdual damage equals 2/3 of their current hit point total, the character becomes exhausted. |
| As with the normal rules, if subdual damage exceeds current hit points, the character falls unconscious. |
| This fatigue subdual damage heals like any other subdual damage in the d20 System. |
Example: Farve of Bjornaer tries to cast a spell of level 33. He misses his roll, and therefore takes fatigue. He takes 1d6 of fatigue per magnitude (33 ÷ 10 = 3) of the spell. He rolls 3d6 and now has 12 points of subdual damage. Since his hit points are currently at 15 (out of a maximum of 22), he is now exhausted. After one hour of rest Farve will then be fatigued.
Size
Size in ArM is uses a Size characteristic that even PCs share. The following table converts these to d20 size measures:
ArM Size |
d20 Size |
|
-3 |
Tiny |
|
-2 |
Small |
|
-1 |
Medium |
|
0 |
Medium |
|
1 |
Medium |
|
2 |
Large |
|
3 |
Large |
Stress Rolls and Botches
Botches are an important aspect of many activities in ArM, particularly magic. Use the following guidelines:
- On a roll that is susceptible to botches (indicated as "d20b" in these rules) a result of 1 or 2 indicates a botch check.
- For the botch check, roll the appropriate number of botch dice and follow the normal ArM rules to resolve the botch.
Botches are modified by the Cautious Sorcerer and Faerie Raised Merinita feats.
Aging
This table summarizes human aging penalties from d20 (there is some variation between the games--these are based mostly on D&D cutoff ages)
| Age Category | Human Age | Effects |
| Child | 0-11 | -3 to Str and Con; -1 to Dex, Int, Wis, Cha |
| Young Adult | 12-15 | -1 to Str, Con, Ins, Wis, Cha |
| Adult | 16-34 | no modifiers |
| Middle Age | 35-52 | -1 to Str, Dex, Con; +1 to Int, Wis, Cha |
| Old | 53-69 | -2 to Str, Dex, Con; +1 to Int, Wis, Cha |
| Venerable | 70+ | -3 to Str, Dex, Con; +1 to Int, Wis, Cha |
| (Taken from Star Wars RPG and D&D) | ||
Aging Rolls
Many Ars Magica rules (longevity potions, longevity charms) depend on aging rolls. To simulate this in the d20 world, allow characters to avoid the prescribed aging modifiers (listed above) by making a save every year, during the winter, after they achieve the base age in any given category.
To make the roll, make a Fortitude save at a DC of 21. Modify this roll by the following:
| +1 for every 5 lab total points in a longevity potion |
| Bonus from any longevity charms (or banes). |
