The endless dungeon is a limitless number of levels, each filled with progressively stronger beasties, which could give players a fun way to get some money/items and exp without having to play through the storyline for the fifth, sixth, seventh, etc time.
The whole point of is to challenge players and provide an alternative from the main storylines; great if you've already beaten the game and have nothing else to do.
The dungeons also would be without teleport scrolls, making running to town for a break impossible.
There would be no definitive end, thus an "Endless Dungeon."
If you can't beat that "impossible" monster at the end, then there's that much more of a reason to build up your character/party and go back to try again.
As it progresses, the monsters get harder.
They keep getting harder up to a point when the are the hardest and they stay that way.
The dungeon never ends, however. So if you can kill the hardest of the hardest monsters, good for you, but they will appear in large groups, and you will then have to organize a plan with your friends to take them in on in smaller numbers.
Just a fun and challenging area for players who seek it.
Such an infinite dungeon would have the following features, as some have mentioned
- Spawning a unique monster every other level as well as a boss on every tenth level, which has a high chance of dropping some decent loot
- You can't use a portal (or, you can use once so that you don't die, but you can't go back down)
- You have to survive only on health potions and what you find on your way down
- The monsters get harder each level and somewhere something really valuable is waiting for you.
Progression in the story line allows you to get deeper in the dungeon, meaning that you can enter the dungeon after the first quests, but you have to find "keys" or complete quests unlocking the dungeon as you delve deeper into the dungeon.
Similar to Fire Emblem Valni Tower (if you know it), or Zelda: Phantom Hourglass, though here this "infinite dungeon" is the main dungeon actually.
Meaning you start playing the storyline, which the dungeon right next to where you started the game, but you can only go to the shallow levels first; you always come back to the dungeon and go deeper, the more you progress in the story line.