So I finished the book Boneshaker by Cherie Priest a while back and am running a game in a very steampunk/necromancy influenced world (I'd call it Necropunk). I was very impressed with the Blight gas from the book so I decided to incorporate it into my world and this is what I've come up with.
The Blight
The blight is a yellowish/green gaseous substance of unknown origins (I use alchemy in my world but it could definitely be magic or just something natural depending on what you're going for). Living creatures that breath in the gas must make a Fortitude Save (DC15) to resist the blights effects. A character that fails his or her save takes 1d2 points of bleed damage per round. Each round the character continues to breath in the blight (s)he must make an additional Fortitude Save, any further failed saves increase the bleed damage by 1 die (1d2 to 1d3, 1d3 to 1d4, 1d4 to 1d6, etc.) and the Fortitude Save DC bt 1. Characters that are killed by the blight are raised in 4d10 hours as plague zombies. Characters that take over half their base HP in bleed damage but do not die receive the Half-Dead Template (see below). Creatures that do not breath are immune to the effects of the Blight. In addition to affecting living creatures the blight also destroys plant life.
Lemon Grass
A strange drug created by unknown Alchemists (this is just my preference), Lemon Grass is a flammable paste like substance made from concentrated blight gas that is usually mixed with tobacco and smoked in pipes or cigarettes. A character smoking or within 5ft. of anyone smoking Lemon Grass must make a Fortitude Save (DC13 + 1 for each additional dose applied) or take 1 point of bleed damage per dose smoked every 1d10 minutes for 1 hour per dose smoked. In addition, a character that fails this save gains +2 Constitution per dose smoked for the duration listed above, this increase grants the smoker +1HP per dose smoked per HD. Characters that die from the bleed damage caused by Lemon Grass are raised in 2d6 hours as plauge zombies. Characters that take over half their base HP in bleed damage but do not die receive the Half-Dead Template (see below). Creatures that do not breath are immune to the effects of Lemon Grass. Finally, there is a 10% chance of contracting zombie rot from smoking Lemon Grass if the character fails his or her save.
Half-Dead Template
Half-Death is a strange state a character can reach when they have died yet remained living. Half-Dead characters receive the following characteristics:
+2 Constitution, -2 Charisma: Half-Dead characters are resistant to disease but have a strange and eerie look about them.
Intimidating: Half-Dead characters appear to be "wasting" and have signs of gangrene around their eyes, mouth, and filangies thus receiving a +2 bonus on Intimidate checks.
Half-Death: Half-Dead characters strange state between life and death gives them odd reactions to positive and negative energy. A Half-Dead character only heals half the amount of HP from positive energy (rounded down) but only takes half the damage from negative energy (also rounded down); creatures with Negative Energy affinity reverse these effects. In addition, Half dead characters receive a +2 on saves to resist any effects that could potentially turn them undead. Finally, a Half-Dead character that becomes undead may replace his or her Charisma score with their Constitution score if it is higher.
it looks good, the only thing I'm confused by is the lemon grass. you describe it as something that people smoke all the time, but it does at minimum 6 damage every time you smoke it as well as adding 1 hp for the duration? I don't see this as a good thing so I'm just wondering why someone would want to smoke this stuff?
Lemon grass is also a real-world cooking ingredient. Maybe that's not going to matter to your group, but I might look for a different name.
I think the blight's also going to get real deadly really fast, since the bleed is presumably cumulative. I might instead just have the DC go up by 1 each round if you want to go for that affect. Also does the bleed heal normally -- for some reason, I feel like it's meant to be internal bleeding, and am not sure that would be stopped with a Heal check... Maybe that's just a bad assumption on my part, though.
Sorry for the delay on the response (I'd forgot I posted this). Lemon Grass is actually meant to have the same name as the real world ingredient, it's more of a street name though. I actually changed it to Lemon Sap though if it's not mixed with tobacco (since it's a paste), though when it is mixed with tobacco (which is usually the case) it's referred to as lemon grass. The bonuses from it aren't really meant to be that great, like most drugs, I may add something else though to make it seem more beneficial though, such as a +2 to Strength or something of that nature.
As for the Blight and the bleed damage, it is meant to be internal bleeding but I was thinking the heal check would still be possible (maybe uping the DC to 20 or 25) but was going to leave it at 15 to start since a cure spell stops any sort of bleeding automatically. I know in the real world your lungs bleeding is far harder to stop than a cut but, since this is a fantasy game and 1 point of magical healing can stop a paper cut (which I imagine deals bleed damage) or a gun shot to a vital organ that I shouldn't really tamper with it too much. That's just me though, feel free to change them as you see fit if you're interested in using it in a campaign.
EDIT: I forgot to mention that I wanted the bleed damage to be cumulative so the blight was more deadly. It's supposed to be a sort of "wipe the board" with lower level NPC's and meant to hurt low and mid level PC's. However, all the parties I've run with that encounter the blight have been at least 3rd level and it usually doesn't effect them too bad. Also, with the few times I've run with the blight I've always allowed PC's to purchase gas masks (I count them as Masterwork Tools) which automatically protect them from inhaled poisons and similar substances.