Many details were revealed about Niflheim during this Final Fantasy XV ATR. This is the country that holds most control over the world in which the game is set. It’s quite different from Lucis, Noctis’ country.
Lucis is trying to get its independence. It’s the last bastion against Niflheim. Lucis is a magical kingdom, drawing power from its crystal. They use magic for barriers to protect against enemies. Niflheim’s empire is more mechanical. It has human and mechanical warriors. It has huge hordes of soldiers and relies on numbers. Lucis does have some mechanical conveniences, like its cars, but it’s primarily poised where it’s about magic against machines.
Iedolas Aldercapt is the supremacist leader of Niflheim. He’s semi-retired, but still active. The one who runs the political scene is Ardyn Izuna. He’s at the center of the empire and quite shrewd. Ardyn expanded Magitek manufacturing. General Glauca is head of the military. He commands the Magitek infantry.
Magitek soldiers are the general infantry in Niflheim. There’s a former all purpose sort of Magitek soldier and a current combat prototype. Both look like people. Their commanders are human, but all the foot soldiers are robotic.
There’s also a female dragoon in Niflheim named Aranea Highwind. She’s a human officer in the military and head of the Third Army ZCorps 87th Airborn Units. They’re a group of mercenaries. She’s officially referred to as The Dragoon. She’s popular with the Square Enix dev team.