As far as I remember, Hagrid (or Albus? Albus,I think; but anyway) tells Harry in PS that Voldemort had used some unspecified Dark Arts to achieve immortality. I think it's logical that his Death Eaters might have been somewhat involved in the immortality-seeking process. The name sorta gives it away. Snape knows how to bring Sirius back. /The final comment was brought to you by a dedicated Snape/Black shipper.