In N Terraria, after the sun goes down each biome has its own night version. Generally the night version is a slightly harder version of the biome. The night variant of each biome may also contain different mobs.

The Hallow, Corruption, Sky, and Forest each have a night version.

Forest Night

Mobs: Zombie, Demon Eye, Night Bat,Skullic Assassin

Hardmode Mobs: Brain Eater

Graveyard Hardmode Mobs: Skullic Guardian,Skullic Fighter