The Underworld is a neverending hellscape filled with all sorts of horrific monsters, but particularly demons and also ice, water, and lava.
The first level is different than the others. It features an oval chamber, within which you will find Ice Father and Fire Mother. Blocking the only entrances to this chamber are seven Day Demons, one for each day of the week. There is only one Day Demon active at a given time depending on the current in-game day. These guardians are each immune to all elements except for one and they each do a different damage type. None of these are types are PHYSICAL, so plan accordingly. In many cases, you won't have any way to hurt the Day Demons and will have to run around them.
- Lunica, Demon of Monday: attacks with MOONLIGHT, weak to SUNLIGHT
- Martolea, Demon of Tuesday: attacks with POISON, weak to ELECTRIC
- Mierca, Demon of Wednesday: attacks with PSYCHIC, weak to POISON
- Joilaca, Demon of Thursday: attacks with ELECTRIC, weak to BLAST
- Vinerin, Demon of Friday: attacks with FIRE, weak to ICE
- Sambaca, Demon of Saturday: attacks with ICE, weak to FIRE
- Dumin, Demon of Sunday: attacks with SUNLIGHT, weak to MOONLIGHT
Ice Father & Fire Mother
This boss duo is one of the hardest encounters in the game. When you enter the chamber, you'll see a pool of lava on the left and a large pattern of ice on the right.
In the center of each is a throne. If you kill either boss, they will simply respawn at their corresponding throne. To defeat them for good, you have to first destroy the thrones. When you destroy both thrones, all seven Day Demons become active and start moving towards you. Therefore, it's best to target one throne/boss before moving onto the other.
If you kill both thrones and both bosses, a portal to Underworld 2 will open.
The Infinite Underworld
Each Underworld level past the first is generated with a random layout, a set of themed monsters, and one or more randomly generated bosses (Barons).
Barons have random appearances, stats, resistances, damage types, spells, and abilities.
Each one drops a demon ring, which will permanently increase your HP and damage. But while you're getting stronger from demon rings, the monsters in the Underworld also scale according to its depth.
One in eight Underworld levels will be a "treasure room". These still contain Barons, but otherwise they are empty of enemies. Plus you can find plenty of gold, chests, and two Hawkers in each of these rooms.