Travel to the Ring of the Law in Blackrock Depths and place the Banner of Provocation in its center as you are sentenced by High Justice Grimstone. Slay Theldren and his gladiators and return to Anthion Harmon in the Eastern Plaguelands with the first piece of Lord Valthalak's amulet.
Heed me well, <name>. For the banner to work in the desired way, you will need to step into the Ring of the Law in Blackrock Depths. Allow yourself to be sentenced by High Justice Grimstone and place the banner firmly on the ground in a sign of defiance.
Your victim will not be able to restrain himself and shall enter the fray filled with a desire to fight you. Once you obtain that which you seek, go back to Anthion.