I have always disliked the poisons for the d20 system.  They just didn't seem like the killers that they are in reality (yes, I know this is a fantasy game, but poisons in fantasy novels kill people, don't they?)...anyways, here are the rules I made up using the real-life LD50 of the world's deadliest snakes.  The LD50 is the dosage where 50% of those injected die (though usually this refers to plump little mice).  It's weighted towards people with 10 HP, so heroic types will still have a higher survival rate.  These venoms would also work well with d20 Modern or d20 Call of Cthulhu.  I realize that some of the snakes (especially the Inland Taipan) are incredibly lethal (yes, it does 267.5 points of damage on average, over 32 hours), but that is why they are so dangerous...give the characters a chance to notice the snake and avoid it.

I admit that the system is far from elegant, but it works pretty well, and venom isn't that common in most games.  Remember when you are rolling dice for damage, you are only taking the highest roll for that type of die (4 for d4's and 6 for d6's) and treating it as one point of damage.  You roll the listed amount of dice at each stage of the envenoming, note that each of the different types of venom (hemotoxin & neurotoxin) roll different types of dice at different stages.

*  Roll an additional die for damage in the stages marked with an "*". 

#  Roll one die each stage ending with the stage that is listed.  Fraction is the portion of 24 hours that the venom will do damage.

q       A creature must hit by at least 2 or it only injects enough venom for half damage.  A critical success injects twice the normal dosage. An individual creature's venom damage is increased by 1 for each point of Constitution bonus (fractional amounts increase at a like rate (i.e.: 1/10 + 2 = 3/10)).

q       After the initial injection and prior to each stage a Fortitude save must be made.  A successful Fortitude save results in half damage (retain fractions until all damage is accounted for).  A modifier of +3 to -3 may be assessed for rest versus activity.

q       Damage is assumed to be spread out over the length of the stage that it occurs during, and may be prorated by the DM.

q       When rolling for damage only count positive numbers, basically just roll the dice and add up the number of 4's or 6's that you rolled and that is the damage for that stage.

q       Once a character has taken any damage from venom they are treated as Fatigued until they heal back to full HP.  If they lose more than half their HP they are also considered Exhausted until they heal up to at least half their HP, at which time they become Fatigued.  This is in addition to the effects listed below.

q       If damage from a hemotoxin exceeds half the character's HP the limb that was bitten is permanently ruined and cannot heal.  The character will always be at half HP until the limb is amputated or magically healed.  After that you still permanently lose 10% of the total damage done from your Con (unless the limb was magically healed).  If the strike was on the torso or head you do not need to amputate, but you will be Incapacitated and you will lose 20% of the total damage done permanently, and if the face was struck you will be badly disfigured.  All these permanent effects assume that no magic was involved in healing or stopping the poison.

q       Damage from a neurotoxin that exceeds half the character's HP will also cause the loss of 20% of the total damage done in permanent Con damage.  Again, this assumes no magical intervention.

q       In both cases the permanent damage is not assessed until the character is healing.  Simply lower the HP you can heal back by the appropriate amount.

q       A fumbled Fort save results in a character being allergic and starting to die when they take the first damage from the bite or sting

 Example:  Risley Dunshill has a Con of 12 and 12 HP, and is bitten by a Fer-de-lance.  Risley tries to make the DC 21 Fort save against the venom but rolls an 8 (+1 for his Con of 12)=9 which is not good enough to halve the damage from the bite.  Risley would be able to make a save before each stage, we will assume for purposes of this example that he does not succeed in rolling a 21 on any of them.  After one minute has passed, the GM rolls 9d4 (Fer-de-lance venom is a hemotoxin), not counting anything but 4's, and rolls two of them.  Risley loses 2 HP and is experiencing burning pain in his leg and the onset of swelling.  After 2 minutes the GM rolls 9d4 again, and gets one 4, thus Risley takes one more point of damage.  After 4 minutes, the GM rolls 9d4 again and gets one more 4; Risley's leg is now covered with blood blisters and his foot is almost twice its normal size.  After eight minutes the GM rolls and gets one more 4, which takes the total damage up to 5.  At the 15-minute mark the GM rolls and gets three 4's, bringing the total to 8, which is over half his hit points, his leg is now completely useless.  At the half hour mark the GM rolls and gets no more 4's, giving Risley a slight respite in which he writhes in pain.  At the 1-hour mark the GM switches to D6's and unfortunately rolls two 6's that brings the total to 10 points of damage. After another hour passes the DM rolls and gets no 6's which means no damage to Risley, however at the four hour mark the roll results in one more point of damage, so Risley is at 1 HP.  At the eight-hour mark, Risley's organs start to shut down and he takes 4 points of damage, which means he is now Disabled.  He stabilizes a little bit and takes only 1 more point at the sixteen-hour stage.  Over the course of the next sixteen hours or so, Risley does not lose any more HP's and has survived the Fer-de-lance bite.  Unfortunately he is still at -3 HP and his leg will never heal without magical assistance.  If it is not amputated he will not be able to heal above 6 HP, and even if it is amputated he will still lose 1 point of Constitution permanently (10% of 15 damage). 

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