You go to Malacath Shrine/Totem and it puts you in queue.

Then he banishes you into a delve like area where you are to fight another mortal to proof your worth and become rewarded with immortality, drink, food, and constant battle(Hypothetically)

He could pit one mortal vs another or even groups of mortals vs other groups.

I think this method would be a simple way to add it in to be lore friendly and without much work since you would just use empty delves as are "battling pits"((So the developers don't have to create arenas but instead just use the countless delve designs we have))


Now everything seems to have a skill line in this game, so of course Malacath Arena's should be a world skill line that only effects arenas.

It should not give any rewards only a leaderboard so you can get a rating to fight players with similar rating/elo, the people you fight can be any faction.