I don’t say this to be mean but if you think Pixel Dungeon is “Ultra Nightmare” mode, games like Nethack or Caves of Qud would probably cause you actual physical pain.

Pixel Dungeon is one of the most beginner friendly roguelikes, widely considered the “next step” in difficulty after the Mystery Dungeon series. (I often recommend it to people that enjoyed Pokémon Mystery Dungeon but find Nethack and DCSS too intense)