So I got the quest and talked to the tavern owner, teleported to the other town and killed the zombies.
But once inside the den I'm just: "What on Earth is this?!"
There are 4 portals in the room that are constantly spawning new demons.

Aside from breaking the bank for health potions is there even a way to complete the quest?
Is there only a set number of demons that are supposed to spawn (the numbers monster in the quest area keep lowering and going back up when I kill a demon and another spawns that I can't use that to track them well) or a way to close the portals to stop them from spawning more demons?

Any help or insight would be appreciated.

Hi, until someone with expertise shows up, I may hint at this video: https://www.youtube.com/watch?v=K5wZlB_sDWw
In particular, pay attention to minute 43:00. Maybe this helps.

I guess you're supposed to be positioned somewhere for the outer pillars to appear?
Not sure if I quite get it and I'll have to go back later when my character won't need more potions than what I can buy, but it might be a clue for when I get back to the side quest in the future.
And there are vidoes of other quests on that chanel too, so thank you for the resource. :D

Finally figured out where to put the map for Treasure side quest in TH2 (unlike the places where you can exit the dungeon to return to the town to the right of the church, the quest entrance is on the left and will only open if you place the map on it), but it had a ton of fire spitting demons, so it's another quest I'll only be able to return to much later. XD

On the plus side, it seems to be a normal quest area with a fixed amount of demons to kill. :)
(From what I could see after a hasty retreat anyway. XD)

Figured I'd share in case anyone else found the quest to be a bit confusing initially.