Need help finding the title of a book...?

I read a book a little over 3 years ago, and the title is a little fuzzy to me. At first I believed the title was Moonstone, after Yahooing and Googling I could not find what I was looking for, and to be honest I don't really know what the book was about but I will relay what I remember. First is a name either Talesin or Myrddin, then the next thing that i remember is an antagonist that was "immortal" who at one time had in his possession the Holy Grail. The last thing that I can remember is that the main characters who I don't know the names of got sucked into a portal where at the right time they saw Camelot and that was at the end of the book. If anyone can decipher this and understand what I am asking I would be very thankful and if you help me out I might just be inclined to send something to you for your help