|Adapted from here: http://www.greatbasinart.com/custom-obsidian-knives.html|
It is not in combat that the Obsidian Knife shows its most insidious power however. If a PC is in a position of having a helpless human, demi-human or humanoid victim in her/his power (a bound prisoner, an unconscious enemy or even a sleeping friend), then the PC must Save v Spells or be compelled to slay the powerless individual with the Obsidian Knife. If the victim is a friend (another PC or NPC member of the party) they can Save at +2.
Should the Save be failed (or, if the PC just wants to kill the helpless individual), the PC automatically succeeds in killing the victim. The PC temporarily gains 1hp for every HD that the victim possessed, even if this takes the PC above their current maximum. Round to the nearest HD (ie, 1/2HD, 1-1HD, 1+1HD all equal 1HD; 5+5HD and 6-4HD both equal 6HD). These hp are the first to be lost in combat or through other injury and cannot be Healed. At the next dark of the moon, the PC will fall unconscious (and unable to be revived) at sunset, and lose d4hp (no Save is possible). When they awake at sunrise they will have a cut on their body as if from the Obsidian Knife. If by any remote chance they are naked and being observed during their catatonic state, a gash without any visible cause will appear on their skin during the night. The next night, the same thing will happen, and every night following until the full moon, or until they have lost twice the number of hp that they gained from the magic of the Obsidian Knife, whichever is sooner, even if the extra hp have already been lost. Because the 'attacks' happen every night, there is no respite allowing regular healing to take place. In order to heal properly, some healing magic must be used over the days after the dark of the moon - unless the PC can tough it out and let the Obsidian Knife's magic run its course.
Example: Jord the 2nd-Level Fighter (11hp) has used the Obsidian Knife to kill a captured Minotaur which has 6HD; he gains 6hp, taking his total temporarily to 17hp. He looses 5hp in a fight before the next dark of the moon three nights later, and is then on 12hp. Having gained 6hp in total, he 'owes' the Obsidian Knife 12hp. That night, he loses 2hp. The next night, he loses 3hp. The following night, he loses 1hp. The night after, he loses 4hp. He is now down to 2hp, and could die the following night.