GURPS has several disadvantages to create characters that feed off of blood. Today I'm going to talk about a few.
Feeds on Blood as a Dependency
Being dependent on blood (any blood) will be considered a Very Common rarity and thus costing -5 points as a base. Human blood would be considered the same, but illegal (without a license) and thus costs -10 points as a base.Adding the interval modifier gets you how often you need your dose . . . but what does that mean? Let's say you have Dependency (Human Blood; Illegal, Daily) [-30] - that means you need to ingest human blood every day or suffer 1 HP per hour after missing a daily dose . . .but how much blood do you need? A taste? A spoonful (of blood helps the undeath go round, the undeath go roundddd, the undeath go round). Seriously, though - how much? It's totally up to the GM I guess. I suggest using a variation of Addiction for those categories:
- Rare (more than 0.5% of average starting wealth)
- Occasional (up to 0.1% of average starting wealth)
- Common (up to 0.5% of average starting wealth)
- Very Common (up to 0.01% of average starting wealth)
Since most wholesale human blood is about $200 to $300 per 1 unit (a bag, or 500 ml or about 17 fluid ounces) of blood (give or take) that means our vampire in the previous example needs about 500 ml of blood per day to stay healthy. If he doesn't get it then he starts taking 1 HP of injury every hour until he does and the damage stops and he can start to heal again.
Feeds on Blood as Draining
A strict reading of Draining makes it seem like Dependency - but a bit different. Use the roles there for determining the value, but if you feed on your required substance you start healing.
Feeds on Blood as Restricted Diet
Using the rules for determining a person’s ST based on their weight we get the following formula:
Volume of Blood in Pints = (Character Weight)/15
Continuing this, loss of 20% of your blood by volume causes hypovolemic shock. Loss of more than 30% requires a transfusion and anything over 40% is fatal unless the subject is rapidly treated.
Using that formula we can then figure how much blood loss you might have suffered from damage (or how much damage you suffer for a given amount of blood loss).
Damage from Blood Loss = 90 x (HP) x (Blood Loss Volume) x (Average Weight)/(Character Weight in pounds)
Blood Lost in Pints = (Damage from Blood Loss) x (Character Weight in Pounds)/(90 x (Character HP))
For example, if a target weighs 200 lbs. and has 12 HP then the total volume of blood in pints is 13.33 pints. So if he takes 8 points of damage the number of pints he’s lost is: (8 points of damage) ¥ (200 lbs.)/(90 x (12)) = 1.5 pints – about 11% of the total blood – enough to send him into shock! If that same man were to lose 5 pints of blood, he would have taken about 27 points of damage (90 x (12) x (5 pt.)/(200 lbs.)).