Demons and the Sun

Sunlight is one of three things that can kill a demon. The day/night cycle directly shapes when and where you fight.

Spawning

Demons spawn at night or underground where light levels are low enough. There is also an optional Progressive Spawning setting — disabled by default — that restricts demon spawns to players who have learned a breathing style or carry Nichirin items, preventing new players from encountering demons before they're ready.

Sun Damage

Any demon in daylight takes massive sun damage and burns. A scrap of shade is not enough: a lone block overhead, a small porch or the spot just inside a doorway or window still lets the sun reach it. Even the strongest demons die in seconds under direct sunlight — a demon wearing any helmet is fully protected, following the same vanilla mechanic as helmeted skeletons. Demons can pick up dropped armor, so be careful about leaving pieces near them.

If a fight drags past dawn, kite the demon into open sky and let the sun finish the job. Deeper inside buildings and caves the sun no longer reaches, so you get no such advantage there.