In Terraria, after the status message 'Impending doom approaches...' appears, it signifies that the Moon Lord, the final boss of the game, is about to spawn. This happens one minute after the message appears. The Moon Lord can be spawned by destroying the four Celestial Pillars or by using a Celestial Sigil in any world where Golem has been defeated. If you're using the Celestial Sigil, the Moon Lord will spawn in just 12 seconds after the message. So, get ready for a thrilling fight!