A stream I saw was showing this method and a lot of people were confused as to how to make this farming spot work so I went and figured it out and then made a video about it
Again, 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.
This is a potential afk farming spot aswell, so have fun and good luck!
A++, getting about 1b per hour, and about 1 legendary per hour. Dropping sentrys along with maraud's pet army is enough to kill everything passively.
Some insight to add-
When your moving the purple out of the way, some of his adds might also tag along, make sure you kill these. There's about 6 total that spawn I think, (2 south,2North,2west) and the new ones only spawn once the current have been killed as far as I can tell. This also means that the faster you kill them, the more exp you're going to potentially get.
Just did it for around 20mins on T6 and got over 500million xp, looks good (for now)
i'll add that the champion needs to be kited a fair distance away, my first try i only moved him a few screens away and he teleported back when i engaged the adds.
I'll keep doing it while watching some videos but it's pretty boring and with no gold or regular loot drops if you are geared enough to efficiently run T5/6 bounties i'd recommend them instead if you want to play actively.
Because I make the videos for other people, not only myself
I made a video about CoTA runs, Core runs, Hive runs, and a couple others, all methods that I absolutely hated doing but my viewers might find helpful or interesting. I try to help people play the game in the best, most efficient manner possible, so if that means doing something that I find extremely annoying and repetitive, I will.