Braies of the Tides
The Braies of the Tides are a not uncommon magical piece of clothing owned by wealthy and infuential women. They are made of high quality linen, silk, or some other expensive material. They are always a dark color, with black or dark blue being the most common. These are the most expensive of the unmentionables that a woman might own, barring Bras of Holding and the like.
Braies of the Tides have two magical properties, they are more durable than traditional cloth, and the fabric has an almost unlimited ability to absorb blood.
An Inconvenient Truth
An often overlooked truth in fantasy literature and gaming is that once a month women have a certain visitor. Modern aspects of feminine hygiene such as liners and such were not invented until the last century or so. Vampiric Panties, or Braies of the Tides are a magical answer to a very common event.
Additional Ideas (1)
Plot 1: A mysterious bout of venerial diseases has suddenly come to plague a city. Spreadding like wildfire and un-talked about by the community. A faulty manufacturer of the panties could be to blame.
Plot 2: Some pervert has been stealing into the chambers of lords and ladies and sniffing their panties! Finding the criminal reveals he has been able to reverse-engineer the panties to give up blood, which he uses for more sinister necromantic purposes like creating hollow children.
Plot 3: The rivers of a far away town begin to run red with blood, destroying crops, causing disease, and fouling the water for drinking. This happens to coincide with a certian fad in panties several miles away. All the blood had to go somewhere, how can you convince the wealthy people of a far away town to care about this hamlet?
Plot 1. How about replace the vampire panties with any magical clothing, and change the venereal disease to a skin disease such as leprosy. There you have it, albeit less sexy, the same basic plot... no panties.
Plot 2. My favorite of the three. Of course why menstrual blood is a specific ingredient to a necromancer's abilities is beyond me. I think that there are better plots than this one, that have done far worse. For example, the Species movies and wasn't there an Angel episode where women get pregnant from a demon? Not to mention Blade and Twilight that both involve a half vampire from a pregnant woman. Your plot would definitely keep the PCs guessing, which is good.
Plot 3. This basic plot which I would call 'the messenger of doom'. This plot is where the PCs have to bring the message to a people that they are the cause of someone else's misery, That through ignorance or whatever, they are cursing someone else. Ultimately, I find these argumentative style plots yield very little by way of enjoyment for the PCs and try the a GMs reasoning skills for multiple NPCs like very few other situations do. I've seen a few campaigns crash over situations like this one. The key is evidence. What evidence is there that the panties are the problem?
Anyway, I will raise my vote but you will need to fix up Plot 2 or 3 to get me to take back my original comments. (Also in hindsight, I never should have issued such a challenge now I feel that I need to keep talking about this when I was tired of talking about it a long time ago.)