Tome of the Cabal
Speak with Krom Stoutarm in Ironforge.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 Krom Stoutarm can help you with this. An itinerant warlock these days, he spends his days hoarding ancient texts and skulking about the library in Ironforge.