Tome of the Cabal
Speak with Jorah Annison in the Undercity.Description
It will take more than a simple summoning circle to bring forth a felhunter for you to challenge.My three acolytes and I will assist in creating the magic circles. You require a copy of the Tome of the Cabal, which has been lost to us for some time. It was written by the first group of warlocks to summon a felhunter unaided.
Perhaps Jorah Annison can help you with this. He's employed as a cartographer in the Undercity these days, but still knows more of ancient texts than any I know.