This is a dagger carved from obsidian, used in pagan times by a cult of witches to cut the hearts from their wretched victims. The secrets of performing this dark ritual have been lost, but dedicated research might reveal them.
If the ritual is performed properly on the correct evening, six pawns of Corpus vis can be gathered from the heart of the victim. This vis may well be tainted with the Infernal, and is certainly ill-aspected, being the wicked gains of the act of murder.
Only women were called to the cult that made the dagger, and it was never meant to be used by male hands. Males who possess it will soon find themselves adopting female trappings and mannerisms, and will eventually be driven to emasculate themselves. If a man actually attempts to enact the ritual, it will succeed as normal, but he will be driven mad for a moon, and will attempt to slay additional victims with the dagger, without regard for personal loyalties or for the fact that such additional victims will not produce any vis.
If the knife is left in storage, it will call out subtly to nearby females, who will be driven to take the knife and use it for its intended purpose.