The Undying

It appears in a puff of sulfurous yellow smoke. It is like a beetle with a golden shell and six clawed legs. The eyes are like three glittering gemstones of fiery red. It fills the hand that holds it. But non other than the summoner may pick it up or hold it, lest it vanish and appear elsewhere in an instant. It will speak to the summoner and offer a bond - blood for eternal life.  If rejected  it will return whence it came, again in the puff of yellow smoke. If accepted the thing will climb over the summoner's body and anchor itself as indicated by the host, and bite with it's razor jaws and hook in it's clawed feet, anchoring itself into the flesh and becoming one with the host. It can not be cut away. And nothing that harms the flesh may harm the Undying.  I, Vashti of Bijhat have summoned it and bound it to me and it fulfills it's agreement- i drink poison which makes me ill but do not die. I am stabbed and do not die.  I am immortal and shall rule forever!

Book of the Immortal. Vashti King of Bijhat. 1178 T

Summary

The spell that summons the Undying is found within the Crimson Tome, an ancient book of demonology.  Once summoned the entity makes the offer of blood and life.  If accepted the thing becomes a life long parasite that bonds to the host and causes one wound that never heals.  It bestows no other power than undying life.  The host is immune to the deadly effects of poisons, gasses, wounds, age and disease as promised. 

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The host may not die so long as the parasite is bonded to them and even if chopped to pieces still lives, and might even heal if the parts are joined or laid together.  Pain, illness effects and aging still occur. The host just can not die so long as there is anything of the body.  Burning, acids or magical disintegration can end the bond and bring death.  Most hosts eventually kill themselves by one of these methods or have others do it as they become fragile, immobile skeletons after a century or two.