Tar-blood Draft, A System-Agnostic Magic Item

Tar-blood Draught

Description:  A small glass bottle with a snug-fitting stopper carved out of bone.  Within is a thick black liquid that smells faintly like fresh blood.  Radiates a faint aura of transformative magic to magical senses, and any alchemist will easily identify its expected effects.  

Powers:  When drunk, the user's blood is magically transformed into a black fluid much like the potion itself for the next 1d6 x 1d6 minutes.  During this time they take half damage (round down) from any cutting, slashing, or piercing attacks and are completely immune to ongoing damage from blood loss or effects relying on blood drain, although they suffer normally from injected poisons and the like.  They don't bleed at all, not even a drop, with any wounds sealing over with a smooth, glossy black membrane that forms into more normal scar tissue when the potion effects end.    

Possible Origins:  Tar-blood draughts are an alchemical creation favored by mercenaries, swordsmen, and others who care more for survival than beauty.  Several formulas for the stuff have been independently discovered, with the most common Dwarven version needing orc blood as a component and the most common orc recipe calling for Dwarf blood.  Both can use dragon blood as a substitute (usually at extra expense) and some alchemists claim blood from a bull or similar large, powerful mammal will also do fine.  Those alchemists are invariably neither Dwarves nor orc-blooded, both of whom insist their own concoctions are superior in quality.

Complications:  While the potion is in effect, all healing effects are halved (round down) and any ongoing regeneration is cancelled completely.  

Wounds taken during use will always form scars, and if the user suffers enough damage to drop unconscious they suffer a permanent 5% penalty to all tests for social interactions where their appearance is a factor.  This penalty stacks (to a maximum of -15%) if it occurs multiple times over one's career.  The penalty might be negated by suitable clothing to hide your scars, or even inverted if dealing with a situation where those scars might impress or intimidate those you're interacting with.

Design Commentary:  This stuff's a big help if you're facing enemies with the right types of weapons and worse than useless otherwise. Note that most animal attacks will have their damage halved, but constriction and bludgeoning bypasses the potion's effects.  No creature that's gotten even a small taste of the tarry goop running in a  user's veins wants another sample, which may discourage repeated bite attacks.  Mosquitos, leeches and similar blood-sucking vermin will give a wide berth to anyone who's drunken one of these potions - at least while the effects last.