And how does the story go? You'll excuse me if I simply plunder the quote from Wikipedia:
Blade of the Immortal follows the deeds of Manji, a skilled samurai who has a decisive advantage: no wound can kill him, except for a rare poison. In the past, his criminal actions led to the death of 100 other samurai (including his sister's husband). He becomes immortal at the hand of a 800-year-old nun named Yaobikuni, and is compelled by the death of his sister to accept the quest that will end his agelessness. He has vowed to make amends by killing 1000 evil men, and until he does Manji will be kept alive by kessen-chū, remarkable creatures that allow him to survive nearly any injury and reattach severed limbs even after hours of separation. They work by sacrificing themselves to seal the wound - they're worms that were bred to be as close in their chemical and physical make-up to humans as you can get without being human. They cannot handle regrowth on a large scale, but, for example, can reattach a severed limb or seal a hole in the brain.
Of course the overall picture is more complicated. This quote denotes just the general idea, to get more you'll have to go back to the source. Now, some months ago everything went quiet. After the shakes of the withdrawal passed :) I started the stoical wait for some news; and lo and behold, the wait bore fruit - "Blade of the Immortal" is now an anime series, and here's the trailer I found on YouTube: