Just finished making a video on how this works for anyone that's unclear on how to do it, I'll include a written guide below aswell
You need to do the Siege Runes quest in Act 5 and then go to the Abandoned Siege Camp then to the Battlefields of Eternity at which point there will be two runekeepers that you need to kill. One runekeeper is always the same (and you never need to kill him) and the other runekeeper is random, one of three, the one you're looking for is called Emikdeva who is part of the event "The Demonic Prisoner". In order to find this event quickly, run along the edges of the Battlefields of Eternity.
When you get this event, you pull Emikdeva away from the event and die, leaving him far away, then run back to the event (avoiding aggroing Emideva again). As long as Emikdeva is alive, his reinforcements infinitely respawn but only 6-8 can be alive at a time.