His descendent Thomas Tracy was an early settler in New England.
Our family book says that the local folk in England say that his ghost still howls in the graveyard. I don't know how to source that, but yes, those were gruesome acts, although human nature still has that capacity for cruelty.
I won't say his name, but here is his website, and his name and email contact are on there.
Plantagenet Dynasty: Genealogy and History
I talked to him a lot on the Plantagenet group which has gone quiet for months now; he's an expert researcher, was a great help to me. He took his huge database down from rootsweb. I haven't asked him why, but he had it all sourced, so he would know. His sources were in code, and I should have gotten them all and the code before he took it down, but didn't; we have one American royal descendant ancestor in common. If he can't help you, he will know where to steer you.
Credible sources are critical in genealogy, the primary source documents are best, then on down to family bible notes, etc.
Also, Gary Boyd Roberts is a professional genealogist who has written 2 or 3 books. They deal with royal lineages of 600 and one other one with a similar title only a different number of immigrants. Also has one about the American cousins of Princess Diana, her American ancestry, something like that. I suggest you call your local library and ask them to look him up and the titles in their database; it would be in a special collections department, genealogy section, etc. Gary should have William de Tracy's descendants.
Google books has an open page where you can read about the shriek of William de Tracy's ghost.
Notes and Queries by Martin de Albuquerque
Our Brown Deer had an extensive database on rootsweb, but his has disappeared, too. He's gone quiet lately as well. I might write him an email and ask why it's gone. I had a lot of fun with it, had all the American presidents, got a lot of good info from him. de Tracy might be in his database, too.
Otherwise, it might be in some peerage book. Those I haven't tackled yet, kind of formidable for where I'm at. Most libraries have them.
I don't know what's happened with ancestry.com, but they used to have tons of books, including 1 or 2 of Roberts' books I used a lot, can't get to them now.
Let me know if I can be of any further help. If I find anything more, I'll ping you. I could be descended from him (de Tracy) as well, think I also have Stuart ancestry from Mary Queen of Scots, will never be able to make the leap as it was probably illegitimate, and many of the northern Ireland records were burned. There is one blurb in a PA history I can use and a family genealogy, that's it. I got too frustrated with it and don't have the energy to travel to those huge libraries and pursue it like I ought.
Usually some of that family lore have some substance in fact such as the ghost of which you spoke.