I found this explanation on how dying in D&D 3.5 works.
Coming from 5e, I am a little confused. In 3.5e, do you take damage past zero do you stop at 0(-1) or go straight into the negatives? For example, if I have 5 HP and take 10 damage. Am I at -5 (because simple math) or -1?
It doesn't say explicitly. So it could be homebrewed either way I assume. But what is the real ruling?
5e uses an entirely different system based on death saving throws. 3.5e doesn’t have those.
Instead, you just keep taking damage. Whatever your hp was minus the damage you take is how much hp you have left. In your example, that’s −5 hp. Starting from the same 5 HP, if you had taken 50 damage, you would now be at –45. There is no lower limit.
You die at −10 or lower hp. So that 50 damage attack that dropped you to −45 also killed you. Usually, your hp stops mattering once you die, but in a few cases it does matter.
Note that as damage values increase—and they increase fast—the odds of landing between exactly 0 and −10 hp get smaller and smaller. It is rare for a mid-to-high level character to be in that range—usually they just go from “fine,” > 0 hp, to dead, ≤ −10 hp. That makes feats like Diehard that improve your ability in that range pretty poor choices most of the time.
In D&D 3.5, damage can take you into negative hit points, with normal mathematics. If you're on 5 HP and you take 10 damage, you are at -5.
If you're reduced to exactly 0 HP, you're Disabled, meaning you can take only a standard action (which deals you a point of damage, reducing you to -1), or a move action. It's actually quite rare to be reduced to exactly zero.
If you're reduced to between -1 HP and -9, you're Dying. You're unconscious, and lose a hit point each round. There's a 10% chance per round to become Stable, meaning you stop losing hit point each round. Healing stabilizes you, and recovers hit points with normal math, i.e. if you're on -5 and get healed for 4, you're still at -1 and unconscious. You can of course still take damage while dying, but most DMs consider this unsporting. Matt Colville has a popular quote from a game he played in: "Oh no. The earth elemental stomps on your head, to make sure you're dead."
If you're reduced to -10 HP or lower, you're dead. No death saves or anything. You can technically go below -10 HP, but there's not much point in measuring it because you're dead anyway. If the example character with 5 HP took 15 or more damage, he'd be straight-up dead. There's a house rule which lets you survive at up to something like negative half your hit point maximum, which is helpful for higher-level characters.
A nice way to understand 3.5 dying is to go through the versions in order. In the first version you were dead when you ran out of hit points. Simple, right?
An obvious weirdness about that rule was you couldn't be knocked out. So people house-ruled 0 was unconscious, negative was dead. But that was still pretty brutal -- 1st level wizards with 3HP died to a single arrow half the time. So many GM's house-ruled that just a little bit negative was merely dying. To simulate bleeding to death, you lost a hit point each round. Still pretty simple and somewhat exciting: "I'm out guys, down to -6 HP. You've got 4 rounds to beat these goblins and then save me before I reach -10 and die".
Meanwhile other games (GURPS) were letting you make consciousness checks and death checks. Those were fun so instead of always losing 1 HP, you got to make a roll. Made dying a little more interesting and gave the player something to do on their turn.
As KRyan's answer points out, the -10 rule keeps you safe at 1st level, but at high levels it's easy to be knocked from 4 to -12 in one blow. So people started house-ruling how far negative was dead based on level, or Con, or whatever. That's a lot of numbers and it's messy for the new slimmed-down D&D. And that's when they finally abolished negative HP and used only your death rolls each round to make dying into a completely separate mini-game.
