From the sealed Records of the Emerald Magistrates, dating from the time of Iuchiban's first appearance."The scrolls refer to a Mask, made of porcelaine and engraved with certain symbols which I shall not transcribe here. The Mask was made by the fallen shugenja, Yajinden, who has been sentenced to eternal forgetfulness. The scrolls recovered by our investigators and the Jade Champion suggest that Yajinden had not completed the enchanting of the mask when the cult was destroyed.
The Mask was intended to contain an evil spirit, but rather than animate a corpse as lesser Masks do, the spirit was to be joined to the soul of whoever wore the Mask, forming a 'composite being more powerful than either of its parts, a Demon of This World, not of the Spirit Realm'. The horror would essentially be unkillable, for the demon's power would instantly renew the human's flesh, and the human's mortal shell and mortal soul would shield the demon from the intercession of the kami.
Over time, the Mask would overcome its wearer, and the wearer would become a Oni 'hybrid', a half-caste demon standing midway between heaven and hell."
While Yajinden never managed to finish the Mask during his life, he managed it after his death. While posessing Daidoji Henshu, he completed the mask, and Yogo Gotoki forced Mirumoto Fai Suk to wear the mask.
The Mask Wearer becomes unkillable. Simply unkillable. Every mortal wound is instantly healed by porcelain-like flesh. In game terms, the character regains one full health level every round, no matter what. Well...not every mortal wound - but the Mask can heal anything short of being torn limb from limb by a Dragon. Decapitation isn't fatal to the mask-wearer.
The Mask Wearer gains the Shadowlands taint, at a rate of one point per return from the dead. The Wearer is initially immune to spells which affect demons, but this immunity fades as the Mask takes hold. The Wearer is sharing his body with a demon, and each death gives the Oni a little more hold on the wearer's name...
GM's commentary: Yes, it's BAH-ROKEN and overpowered. It's a story element, not a game one. I didn't include the Mask as an interesting treasure, the Rokugani equivalent of a sword+1. It's there for a reason. Fai Suk had a choice between dying and wearing the mask. Togashi warned him there was only one way out of the mask's power, and Fai Suk's close to losing that escape route too.
Sometimes, it takes a darkness to throw light on a character's features...