How to adapt/convert an ability?

How to adapt/convert an ability?

A person asked me to describe my way of thinking when adapting or converting an ability from another system/game/medium/whatever to GURPS. To me it seems intuitive, but I'll try to expand on my thought process here.

I believe that the magic missile spell from D&D is one of the best examples to use here. For reference, I'll post the SRD entry of the spell here.

Magic Missile

Evocation [Force]

Level: Sor/Wiz 1

Components: V, S

Casting Time: 1 standard action

Range: Medium (100 ft. + 10 ft./level)

Targets: Up to five creatures, no two of which can be more than 15 ft. apart

Duration: Instantaneous

Saving Throw: None

Spell Resistance: Yes

A missile of magical energy darts forth from your fingertip and strikes its target, dealing 1d4+1 points of force damage.

    The missile strikes unerringly, even if the target is in melee combat or has less than total cover or total concealment. Specific parts of a creature can’t be singled out. Inanimate objects are not damaged by the spell.

    For every two caster levels beyond 1st, you gain an additional missile—two at 3rd level, three at 5th, four at 7th, and the maximum of five missiles at 9th level or higher. If you shoot multiple missiles, you can have them strike a single creature or several creatures. A single missile can strike only one creature. You must designate targets before you check for spell resistance or roll damage.

Step 1. Discard all the unneccessary information.

Evocation [Force] Do we care about that? No, we don't.

Level: Sor/Wiz 1 This is actually somewhat relevant just to determine the power source. This is a wizard/sorcerer spell, so we will keep in mind that this is an arcane spell and hence should have Magical, -10%. But level? Miss me with that.

Components: V, S For a "default" sorcery spell, this does not matter, because there are Alternative Rituals (page 9 of Sorcery, iirc). However, if you disallow that rule to enforce incantations and/or gestures (I often do that, for example, for wizards), then these components will indicate the need to apply Requires Magic Words, -10% and Requires Gestures, -10%.

Casting Time: 1 standard action D&D actions <> GURPS actions. The "default" sorcery spell takes a second to cast. You could vary it, of course, if needed.

Range: Medium (100 ft. + 10 ft./level) Just keep the range in mind without focusing too much on the exact number, and disregard scaling as it depends on something that does not exist in GURPS - caster level.

Targets: Up to five creatures, no two of which can be more than 15 ft. apart Think about this this way: Yeah, it can target up to five creatures, but it also takes a standard action to cast. GURPS is more granular, so these five targets might've been five separate castings. Also, the number of missiles also depends on the caster level, and we do not care about that. Our magic missile will target a single target.

Duration: Instantaneous This is not a cyclic spell, it strikes the subject once.

Saving Throw: None This might be necessary.

Spell Resistance: Yes D&D Spell Resistance and GURPS Magic Resistance work very differently. I suggest to just disregard this.

A missile of magical energy darts forth from your fingertip (This indicates that it will use Innate Attack (Projectile)) and strikes its target, dealing 1d4+1 (Never look at the damage numbers! D&D damage <> GURPS damage, so you should simply disregard these numbers. Also, this allows you to make fewer conversions. For example, if you compare the D&D spell list with the spell index on my blog, you will see that many spells are missing on the blog. But many of them actually are just equivalent to higher level versions of other spells. GURPS does not have spell levels, so by converting one spell you often cover multiple similar spells. Also, this damage will ignore armor in D&D, but armor works very differently in GURPS, so try to remove the idea of energy attacks ignoring armor from your mind instead of slapping Cosmic, Irresistible Attack, +300% on every damaging spell) points of force damage (This is actually important. Force damage can affect incorporeal beings, so we should keep the Affects Insubstantial, +20% enhancement in mind).

    The missile strikes unerringly, even if the target is in melee combat or has less than total cover or total concealment. Specific parts of a creature can’t be singled out. Inanimate objects are not damaged by the spell. (This is where people normally layer fifty Cosmic enhancements to make it not require a skill roll to hit and disallow active defenses. This is silly. Close your eyes and distance yourself from the D&D mechanics. Imagine what the magic missile looks like and what it does judging by the description. It's a homing missile. Simply add the Homing, +50% modifier instead of doing all the Cosmic silliness. This probably is the most imporant part of converting content - convert the concept, not the mechanics.)

    For every two caster levels beyond 1st, you gain an additional missile—two at 3rd level, three at 5th, four at 7th, and the maximum of five missiles at 9th level or higher. If you shoot multiple missiles, you can have them strike a single creature or several creatures. A single missile can strike only one creature. You must designate targets before you check for spell resistance or roll damage. (Disregard the entire paragraph. We decided that this is represented by multiple castings in GURPS. Optionally, you might add the Rapid Fire enhancement, but I would advise against that.)

Step 2. Find an appropriate advantage to serve as the base. Sometimes, you might need more than one advantage. Most of the advantages can be found in the GURPS Basic Set. GURPS Powers also has a few new ones, but other than that there's barely any new advantages in other books - there's only a few that are very unlikely to be what you're looking for. It is important to look at the mechanical effect of the ability, and not on its description. Try to avoid Create and Control advantages if there is an alternative. For example, if your ability creates an area of noise - use Obscure, not Create Sound. Keep it simple.

    In our example, the magic missile deals damage, so we base it on Innate Attack. But which type? Force is a generic energy damage type in D&D. I suggest using crushing damage type for this spell.

Step 3. Find appropriate modifiers. This might be more difficult than Step 2, as there's a lot of new modifiers spread out across all the books. However, most (not all!) of them are compiled in GURPS Power-Ups 4: Enhancements and GURPS Power-Ups 8: Limitations. One limitation that is important and that is missing from the GURPS Basic set is Reduced Duration. You can find it in Power-Ups 8. If you can't find an appropriate limitation, then you probably can replicate it with Accessibility.

    In our example, we have Sorcery, -15% as the power modifier, Homing, +50% to make it home in on the target, Affects Insubstantial, +20% to represent force damage. Now let's consider the Medium range. 100 feet is roughlt equivalent to 33 yards. Range reduction is not very granular in GURPS, so the closest value we can get is 20 yards by applying Reduced Range, 1/5, -20%. Since it is homing, it moves at a speed of 1/2D yards per second. Currently, our 1/2D is 2 yards. Let's increase it to 4 yards by applying Increased 1/2D, 2x, +5%.

    Also, let's think about the concept of this spell. In D&D, it is supposed to be a weak spell, so instead of letting it scale, let's limit it to just 1d-1 crushing damage. And here's the end result.

    Magic Missile

    Keywords: Missile, Obvious.

    Full Cost: 6 points.

    Casting Roll: None. Use Innate Attack (Projectile) to «lock on».

    Range: 20 yards.

    Duration: Instantaneous.

    A magical force missile flies off your fingertips towards your target. A successful hit does 1d-1 crushing damage. The missile can strike even insubstantial targets. The projectile is homing, so it steers itself. As it uses normal vision to seek its target, it does not ignore darkness penalties. To “lock on,” you must Aim at the target and make an unmodified skill roll. Do not roll against your skill to hit. Instead, use the attack’s skill of 10 – plus Accuracy, if you made your skill roll – and ignore all range penalties. The projectile moves 4 yards per second. For more information, see Guided and Homing Weapons (B412).

    Statistics: Crushing Attack 1d-1 (Affects Insubstantial, +20%; Homing, +50%; Increased 1/2D, 2x, +5%; Reduced Range, 1/5, -20%; Sorcery, -15%) [6].

    I think that it's simple, elegant, and preserves the concept. I don't know if that was helpful in any way, but here you go.