The Morgul Knifing, a botched job?
 

It's just my opionion, but I believe that the Wraith dagger was not originally designed to require a part of the blade to reamin in the wound; If it did then it would make no sense to make it as a dagger, which would leave you with a large (and now unusuable) chunk of magicked metal. My belief is the knife (or knives) worked as follows; those "killed" by it (in the sense of suffering a wound which would cause death if the knife had been an ordinary knife) would become a wraith, rather than simply dying. One would therefore tend to use it to admister not mortal wounds, but fatal ones (say, straight through the heart, or the neck) under which circumstnaces the victim would become a wraith almost istantly. the Afterword, the still intact knife could be withdrawn and used on another individual. In all likelyhood the Witch King in the frenzy of the attack simply missed his mark, hitting Frodo in the shoulder rather than his intended target, most likely the neck, since, movies aside, trying to stab someone through the heart overhand (from what I've read) is almost impossible (unless the first stab was simply to get him to fall wherepon a second stab, upwards, could be admistered). The tip then breaks off, most likey due to the dagger glnacing off of one of frodo's bones (the clavicle most likely given the location of the wound) the Witch King then withdraws the knife to prepare for another try but the rest of the conpany alerted by Frodo's cry reach the scene before it can happen and the WK and company are forced to flee dropping the knife in thier escape. This almost certainly had to be a mistake since had they take it with them Aragorn would have had no idea of the nature of the wound, likely would not have taken such pains to get Frodo to Rivendell as fast as he did, (a minor wound especially one that had clsoed up so soon after would have rasied little concer at first and whn it did migh be interpreted as ordinary septicemia (which Aragorn presumby does know how to keep at bay)

Seems likely to me,

and are the WitchKing's previous victims now the Barrow-Wights?

Gandalf's words seem to indicate the splintering of the blade was intentional.

Many Meetings (emphasis added)

The splinter working its way toward the heart slowly, causing a gradual fading effect, seems to have been the aim all along. Frodo would steadily lose his connection to the world of light, undergoing whatever physical and spirtual changes were necessary. I would think a stab to the heart would have caused simple death.
As for the WK's aiming for a specific part of Frodo, it doesn't seem he would have been trying for the neck. Frodo was throwing himself toward the ground when he was stabbed, so the intended point of penetration would need to be lower than where Frodo actually obtained his wound, in his shoulder.

It still seems like an awful lot of metal to use. If all you need is a splinter why not an arrow, or a dart. Unless maybe it's meant to splinter, but one can take the reaminder back and re-forge it (or sharpen it till it has a point again) and it's re-usable then (in which case dropping it is still a mistake).


In that case, maybe he was aiming for Frodo's back an equally mortal spot. All I was trying to say was that the shoulder seems a lousy spot

You'd think a ranged weapon would have been more practical, since the Nazgûl did use poisoned darts. They nearly killed Faramir with one. Maybe the Morgul-knife's power was derived from the hilt of the weapon, where the evil writing was.

In the hilt, now why didn't I think of that. That solves the whole re forging thing/wasted magic metal thing, you don't have to reforge it; attach the hilt to any blade (we see it on a dagger, but the hilt is probably the right size for a swrod blade as well unless the dagger is fairly small) and it gets infused with the dark power. then when your done just attach a new blade which is fairly simple You also notice that only the dagger's blade vaporizes in the daylight the hilt stays intact .