In Terraria, the phrase 'This is going to be a terrible night' signals the arrival of The Twins, a Hardmode mechanical boss. This status message appears when The Twins are about to spawn, adding a sense of dread and anticipation to the game. The Twins are a more difficult version of the Eye of Cthulhu and must be defeated before dawn, otherwise, they will flee and despawn. So, when you see 'This is going to be a terrible night', prepare for a challenging battle!