Dark dungeon explanation please?

Can one of you explain how dark dungeon works?

I believe the dark dungeon is way for players to challenge themselves to get a hard gameplay and earn a lot of XP. I don't know how TRUE this is but it says chance to earn dark sword but I don't know if that's all real.

In my experience with it I earn more xp at night, 30-40k, then during the day time, 300 tops. It's also the only place I've ever received a powerstone for defeating a boss. If you're fighting enemies that throw blue flame there's likely a good chance that you'll get the Dragon as the last boss and a powerstone for defeating it. All in all there doesn't seem to be a solid pattern to it.

When I play difficulty varies, and it always says level 1 even after completion. Also sometimes I'll get only 20 exp and other times 6000 exp. I've seen no pattern to any of it, and the dark sword is a real thing. Its in the description of dark dungeon in game.