Hero Siege, how can we play on a more difficult setting?

I've noticed that "nightmare mode" exists, but we have only been able to play the intro stage (going back from the main City, so starting it from the end of the stage).
We've found no way to progress past that.
I guess it will be naturally be unlocked with a normal playthrough, but if we do that I'm afraid that the characters will also be of a level high enough to make it no longer challenging enough for our tastes.
Our goal is to increase the difficulty, that's why we were trying to play nightmare mode (~lvl 45) with lower characters (~lvl 15).
If this is not possible, but there are other "legit" ways to increase the difficulty which we haven't noticed, they would still be acceptable.
Best Answer
You have to play through the game to unlock harder difficulties. There are 2 more after nightmare: hell and inferno. There's also a hardcore mode that permanently kills your character when you die.
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How do you increase difficulty in Hero Siege?
Unlocking new difficulties in the game can be done by killing the final boss of Act 6 (Satan). To unlock Nightmare, that is all you have to do, kill Satan in Normal Difficulty. To unlock Hell difficulty, you need to kill All Act bosses before killing Satan.How do you unlock mercenary hero siege?
Purchase one or more mercenary hero slots at the Online Store (maximum 8). Speak to any Mercenary Registrar while controlling a level 20 character. Register your current level 20 character (roleplaying character or PvP-only character), which will make it available as a mercenary to your other characters.How To Progress Into Endgame Difficulty (Hero Siege 2021)
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Answer 2
In theory, you can just beat the final boss of the final act of each difficulty, which shouldn't level your character too much but also get you into the higher difficulties.
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