Nigon is the second underground race.

Excuse me, but why is this race called "Nigon"? There's only one name related to "Nigon" here. And, yes, why a second dungeon? Unless it's just a replacement for the poor Dark Elves. Or, perhaps, it's for a mod called "Heroes 3 on the Heroes 5 engine," creating a proper Nigonian city. If it's going to be part of the fifth installment, then the high-level creatures of Nigon – hydras and dragons – are already taken by the Dark Elves, but you could add harpies, rocs, wizards, beholder, ogres. There's plenty of room for monsters.