Percentage based damage on Instant Death Distintegration Attacks

[Coinage]

I have a weird request. I'll try to spell it out as much as possible.

I was reading up on Disintegration (P118) and Instant-Death Attacks (P118). I was wondering if there was a way of having a Skill roll to reduce damage (a.k.a. a DR roll), in such a way so that the amount of damage you take depends on a % of the roll? So, if you hit a roll of 50%, then you would lose 1/2 of your hit points. Or by the amount you succeeded or failed? Does that make sense?

I guess first I would have to establish the skill level necessary to pass the roll?

Am I making sense? Any advice would help, thx.

[Christopher R. Rice]

Originally Posted by Coinage

I have a weird request. I'll try to spell it out as much as possible.

I was reading up on Disintegration (P118) and Instant-Death Attacks (P118). I was wondering if there was a way of having a Skill roll to reduce damage (a.k.a. a DR roll), in such a way so that the amount of damage you take depends on a % of the roll? So, if you hit a roll of 50%, then you would lose 1/2 of your hit points. Or by the amount you succeeded or failed? Does that make sense?

I guess first I would have to establish the skill level necessary to pass the roll?

Am I making sense? Any advice would help, thx.

There is a modifier in Pyramid #3/9: Space Opera in the Designer's Notes for Psionic Powers that might be of use. It's called Margin-Limited.

[Anaraxes]

Most forumites generally seem to accept pricing a random range the same as the expected value of that range. That is, average for a linear or symmetric distribution. If "hit points" means full down to -5xHT, or 60 points for a human, then a roll centered around 50% of the HP (30 points) is about the same as 30 / 3.5 ~= 9D attack.

[Hide]

Originally Posted by Christopher R. Rice

There is a modifier in Pyramid #3/9: Space Opera in the Designer's Notes for Psionic Powers that might be of use. It's called Margin-Limited.

I will take note of this limitation, thanks!

[Coinage]

Basically, here is my idea.

First, here is my understanding of how a combat round works in GURPS:

1. You roll against your weapon Skill to try to do me harm.
2. I roll against my active Defense to make you miss Me.
3. If I fail, you roll to damage me.

Let us say that Entity X has an "Instant Death Disintegration Attack", a.k.a. IDDA, based on Disintegration attacks (P118) and Instant-Death Attacks (P118). The attack does ∞ (infinite) damage. However, it is a Sense-Based attack. This means that X needs to successfully perceive its target to launch the attack. X wants to against an entity Y.

Step 1. X rolls against Sense skill (or a Comprehension Skill) (B358) to use IDDA on Y.
Step 2. Y rolls Defense (possibly some sort of camouflage, Glamour, Stealth, or something to avoid being seen) to make X miss.
Step 3. Y fails, so X does ∞ damage.

Here is what I want to add to this process for this specific attack.

Step 4. Y gets a last ditch to roll a d100, to determine how much of Y's hitpoints are removed. So, if Y rolls a "36", then 36% of Y's existing hitpoints are removed. Y gets to modify this roll with various advantages like "Luck", "Extra Luck", and so forth.

I wanted to add this extra step to avoid an Insta-kill, and to give players a chance to survive.

What do you think?

[Anaraxes]

I wouldn't think of that as adding an extra step, so much as modifying the rules by which the attack operates. It doesn't do infinite damage; it does a variable amount of damage based on a damage roll, the scale of which takes the target's HP into account.

The touchstone for an attack that just kills someone would be an Affliction of the Heart Attack condition. In terms of damage, a disintegration effect is usually taken to be the amount of damage to take the target to -10x HT -- or, for a humanoid target, 110 points, which works out to a 33d Innate Attack (or a 0d+110 attack, if you prefer). Disintegration is usually treated as the Corrosion damage type.

I'd price a Limitation of a d100 roll for damage, treated as a percentage, times the base value of the attack as a -50% Limitation -- in other words, half the price of the attack, because on average it will do half of its rated damage. I'd use the flat value of 110 points in this case to avoid two rolls. Might even tweak the cost down to make it 100 points, just so the d100 becomes a damage roll with no need to multiply. A little less lethal, of course.

Basing the damage total on the target's HP rather than a pre-determined amount calls for an Enhancement, IMO -- if I allowed it at all. That effect lets the attacker potentially one-shot any tough opponent (dragons, Ogre tanks, gods), which means the campaign is going to be full of BBEGs that conveniently are immune to the attack for various handwavy plot reasons. Occasionally that effect limits the damage, if you're smashing small things. But that's less risky in the first place. So, probably +20% at least, maybe more if I expect the campaign to feature lots of big tough opponents rather than swarms of little ones (say, the classic "Against the Giants" series of D&D modules...)

Sense-Based (B109) means that an attack affects the defender via one of his own senses. It doesn't mean that the attacker has to make some sort of Perception roll to hit. Sense-Based is a "penetration modifier". Its primary mechanical effect is causing the attack to ignore DR. (An attack that did infinite damage wouldn't need to ignore DR; no finite amount of DR would reduce its damage at all. So why pay for the Enhancement?)

There is a Limitation for an effect that requires the attacker to be able to sense his target ("Sense-Based, Reversed", Powers p105. This Limitation only applies to abilities that normally ignore DR anyway. (Otherwise, it wouldn't be a Limitation.) You could apply both Sense-Based and Sense-Based, Reversed to the same ability, meaning that the attacker would have to see (etc.) the target and the target also see the attacker. You might want both on an ability where both characters have to lock their eyes, as with the Harry Dresden soulgaze.

Most attacks require the attacker to be able to sense his target with a targeting sense; see, for example, the penalties for darkness or Blind Fighting. Generally that shows up as a targeting penalty, not a separate roll.

[Coinage]

Originally Posted by Anaraxes

I'd price a Limitation of a d100 roll for damage, treated as a percentage, times the base value of the attack as a -50% Limitation -- in other words, half the price of the attack, because on average it will do half of its rated damage.

Thx. I was looking for something like this. So:
Limitation: roll d100 to determine what % of the target's HP are removed [-50%, since the average roll of a d100 is 50.]

Originally Posted by Anaraxes

I'd use the flat value of 110 points in this case to avoid two rolls.

What two rolls are you referring to?

Originally Posted by Anaraxes

Basing the damage total on the target's HP rather than a pre-determined amount calls for an Enhancement, IMO -- if I allowed it at all. That effect lets the attacker potentially one-shot any tough opponent (dragons, Ogre tanks, gods), which means the campaign is going to be full of BBEGs that conveniently are immune to the attack for various handwavy plot reasons. Occasionally that effect limits the damage, if you're smashing small things. But that's less risky in the first place. So, probably +20% at least, maybe more if I expect the campaign to feature lots of big tough opponents rather than swarms of little ones (say, the classic "Against the Giants" series of D&D modules...)

You're right. I should have been more specific. I would definitely not allowe PC's to have this ability. This was to be applied for NPCs only. It was to create an obstacle for the PCs to get around.

Where did you get the +20% cost for the Enhancement?

[Anthony]

Rather than being always a percentage of hp, I'd probably just go with a fairly large attack with a limitation 'max wounding X% of HP'. Depending on the value of X, this probably ranges from -5% to -40% (no wounding is -50%).

[Anaraxes]

Originally Posted by Coinage

What two rolls are you referring to?

A damage roll, followed by a percentile roll to multiply. If you build a 33d attack plus this percentile limitation, when you hit you'd have to roll 33d*, add them up to get a damage total, then roll the percentile, then multiply to find the actual damage. It'd be much easier to build the attack with flat damage -- saves the damage roll.

In your case, you really want to use the target HP as the flat damage number.

Where did you get the +20% cost for the Enhancement?

Pure feel -- plus an attempt to keep it simple to get what I thought you were looking for :)

To be more mathematically proper about it, the amount of damage you pay for should be an upper bound and the basis of the calculation, regardless of the HP of your target. If you pay to do 100 points of damage, then you shouldn't be able to disintegrate a 10,000 HP target, no matter how lucky you get on the percentile roll. The best you could do is 100 points. If the possible HP ranges you expect to run into are reasonably close to each other as well as the designed damage range (say, it's an NPC ability and as GM you're pretty sure that all the target PCs are humanoid), it won't make a much difference. If a PC buys the ability as a 5 point max for a few points and then wants to disintegrate the Earth if he rolls 00 "because it's based on target HP", then something's clearly wrong.

One way to handle that problem is simply to require that the ability be built to exceed the HP of any target, which in practice will mean taking the lesser of target HP or the ability damage cap.

The rest of that number is just reflexive paranoia for an ability that (in PC hands) smells like it's meant to short-circuit adventures. Being able to insta-kill the final boss is more valuable than being able to insta-kill the hall trash, even if their HPs are close, and it's likely more detrimental to the narrative part of the game. I'm not even sure I'd allow an ability that has a 1% chance of giving the climactic fight an abrupt, boring, disappointing ending, so I don't really have a good feel for how much of a surcharge would be useful to dissuade munchkins while not overly penalizing the players that have to have an ability for concept, even if it's not maximally point-efficient.

--
* probably as 6dx5+3d, or just buy 6dx5 or 6dx6, but even so, it'd be a lot of extra die rolling. You'd want an electronic die roller with decent custom scripting ability to get it to do 33d6 * d100 / 100. A flat damage number saves the rolling and adding of dice of damage.

[Coinage]

I was thinking of just adding a flat +300% for the "Cosmic" enhancement, specifically the "Godlike Tricks" (from Power-Ups 4, p. 7). This means that the damage roll is based not on a preset number, but based on the target's HP.

Originally Posted by Anaraxes

The rest of that number is just reflexive paranoia for an ability that (in PC hands) smells like it's meant to short-circuit adventures. Being able to insta-kill the final boss is more valuable than being able to insta-kill the hall trash, even if their HPs are close, and it's likely more detrimental to the narrative part of the game. I'm not even sure I'd allow an ability that has a 1% chance of giving the climactic fight an abrupt, boring, disappointing ending, so I don't really have a good feel for how much of a surcharge would be useful to dissuade munchkins while not overly penalizing the players that have to have an ability for concept, even if it's not maximally point-efficient.

You're absolutely right to be worried about the potential abuses for this power. This is why I'd keep it restricted to NPC's. No PC's could have it. I would also have a whole bunch of Limitations attached to prevent the advantage from being used to bow up the planet. Like, a REALLY strict Accessibility Limitation.

Any thoughts?