Body, injury, size, monsters, benchmarking

Famously, your ability/competence doesn't decrease as you lose HP. Lots of games have tried to model injuries and death at low/zero HP; here's my shot at it. We'll add a new stat to people and monsters, Injury Points, or IP, scaled to their size.

System assumptions

HP doesn't represent physical durability or martial fitness directly; it represents protagonism in a genre that privileges those things. Thus as we increase in HP (via leveling) we normally improve our attack bonus, but we also improve our ability to ignore various harms and annoyances. If I've got high HP, it'll take me ages to starve to death, not because I have secret caloric reserves, but because, holy mackerel, when was the last time you read a good pulp fantasy novel where the hero starved to death? Similarly, I'll last for ages in combat, taking many more hits than a normal man, and it's not because I'm more physically durable, but because I am a hero, and heroes don't die in piddly melees.

When I run out of HP, I'm in real trouble. Then starvation, cold, and disease are big problems. Every hit counts, and I'll track the consequences of those hits as real injuries.

Who has HP? Anybody with a name, for starters. If you've got a name you're level 1 with 1d6 HP.

Any monster or extraordinary foe has HP, and more of it, the more monstrous they are. This doesn't mean "the bigger they are", which is one of my problems with traditional HP; it's ridiculous for a lich and a stegosaurus or whale to have the same HP just because the latter is so big. The average stegosaurus or whale has no dramatic privilege at all. (Moby Dick, on the other hand, has dramatic privilege in spades. Level 6 at least.)

Effects of IP loss

Reduce IP like HP, when you get hit in an attack or drink poison or whatever. When you run out of IP entirely, you die.

Every time you lose 1/6 of your species' maximum possible IP, rounding up, make a physical save or go into shock and freak out a bit. This isn't a scrape, this is a serious wound. Also, roll on a wounds/critical hit table, or make something up, to describe the injury. Ignore any result like "instant death" unless you've run out of IP, but feel free to lose limbs, break bones, bleed internally, etc.

Keep track of these individual injuries. For each injury, add a penalty die to all your actions, plus whatever other affect the injury would in fact have.

Recover IP at 1 a day, unless you have some super healing ability. When you recover an injury's IP, stop taking the penalty die for the injury, but don't erase the injury from your character sheet until it's actually diegetically healed. If you've lost a limb, it's still lost, even if you recovered the IP!

Calculating

Humans have 1d6 IP, let's say 1 IPd, and have an average mass of 60 to 80 kg; say 70 on average. Let's scale IP to the square root of mass: sqrt(70) ~ 1d6 IP.

This approximation is going to badly short-change small creatures, because I put the entirety of normal human variation in the range of 1 to 6, and I'm not going any lower than 1! It also badly shortchanges the smallest large creatures; the smallest bear is not 1/6 the size of the largest bear.

In practice you won't have to actually calculate the square root of anything; you can just use the following guidelines, derived below.

Smaller than a wolf -> 1 IP

Wolf-sized -> 2 IP

Human-sized -> 1d6 IP (For PCs, feel free to give bonus/penalty dice based on Body/Con mod, but probably don't bother for anybody else)

Ogre/grizzly/pony-sized -> 2d6 IP

Polar bear/horse-sized -> 3d6 IP

Giant-sized -> 7d6 IP

Elephant/T. Rex-sized -> 10d6 IP

Whale-sized -> 15d6 IP

Examples

A horse weighs 200 kg to 1000 kg, with a riding horse at 500. Sqrt(500) = 22 -> 3d6 IP.

A coastal Alaskan male grizzly bear weighs 389 kg. (Actually, look at the source document. N = 5 for male coastal Alaskan grizzlies. Not sure I trust this number. And who taught the data scientists here about sig figs?) Sqrt(389) = 20 -> 2d6 IP.

Polar bear males weigh 300 to 800 kg; before hibernating they err on the bigger side, up to 50% above their post-hibernation weight. So let's estimate a polar bear at 700 kg. Sqrt(700) = 26 -> 3d6 IP. Largest terrestrial predator in the world nowadays.

A fully-grown male African elephant weighs 1700 to 2300 kg. 11d6 IP. Largest terrestrial animal in the world nowadays.

A T. Rex probably weighed 8400 kg. Sqrt(8400) = 92 -> 11d6 IP. Largest terrestrial predator ever.

The largest terrestrial creature ever is probably this hornless rhino thing I've never even heard of before; it had a prehensile upper lip or proboscis, gross! Anyway it weighed 15k to 20k kg. Sqrt(17500) = 132 ->16d6 IP.

Rhinoid creature, courtesy of Wikipedia. Look at this bozo!

Upshot

For fun, we can re-calibrate some AD&D animals by replacing their HD with IP and seeing how many levels that leaves them.

An African elephant has 11 HD in AD&D. That turns into a level 2 monster with 9d6 IP. I don't think that's insane. The elephant has a great defense score; armor as plate, pretty much, since nothing short of a bullet penetrates it easily. So -1 TN for the attacker there. If you're attacking from the front, -1 TN for form superiority, unless you've got a really fancy martial art, since the elephant is going to out-distance you with its tusks and trunk. -1 TN for its size, you just can't reach any vital spots. Probably +1 TN since it can't effectively dodge you unless it's running (at 25 fucking mph! Damn!) in which case you have lost it (and it gets some bonuses from a charge). So you're hitting on 1/6 if it's standing still, 1/8 or 1/10 if it's running. Meanwhile it's trying to hit you, and it's got full penetration and probable form superiority on anything except plate. So it's hitting you at 5/6. Since it's so big, it can probably engage with multiple foes, let's say 2 or 3 at once in between kicking, goring, and poking with the trunk.

Bears have around 6 or 7 HD. 2d6 IP, level 4 or 5, that sounds good to me too! It can only attack one guy at a time, but unless you've got a great martial art, it'll attack with form advantage, and it'll shred through non-metal armor. Penetration along the lines of a spear, I think; shorter claws but more force.

A T. Rex has 18 (!!!) HD, certainly too many, but if we take off the 11d6 IP, we're left with a level 7 monster. That's more like it. A level 7 monster is stronger than Conan in terms of narrative privilege. Fair enough, in Red Nails he can't directly engage a dinosaur (he calls it a dragon) to get away from it, and I don't think he even kills it, just stuns it and escapes.