Underworld Idle Build a demon empire while you sleep

The core loop is simple and strangely satisfying: higher-tier units produce lower-tier ones. A greater demon spawns lesser demons, which spawn lesser demons still, and the same trick repeats across the undead, drowners and warlocks. You start with a handful of resources and a short list of units, then watch that list stretch out while the numbers go from modest to absurd. Four factions share the work: demons, undead, utopians and blackseekers, each producing its own resource. Part of the game is deciding which faction to feed and when, because they lean on each other. Unlocking a new creature is its own small reward, and the pixel portraits give each one real character.

Progress here is not one straight road. When a faction stalls, you sign an Infernal Contract: blow the whole thing up, reset to nothing, and come back with permanent bonuses. The first reset hurts; by the third you are gleefully pressing the button. Between resets, an energy system gives spent time a use. Energy refills on its own, and you can pour it into gems, souls or production boosts, so returning after a break means a satisfying burst of spending rather than a pile of idle gains. A daily in-game clock hands out useful tokens and resources too. And when you really hit a wall, the library holds a grimoire of dark spells, the lab stocks energy crystals, and the junkyard has its own part to play.