Read Isaiah 21:11-12
Isaiah shifts his message to state that Edom has called to the sentries, “Sentry, how soon will the night be over? Tell me how soon it will end”(v.11). Remember Isaiah mentions the watchmen whom he has appointed who brought word of Babylon's destruction. And now Edom is asking God's watchman, Isaiah, when will the night end.
The night here represents the darkness and horror of God's judgement, the vision that terrified Isaiah. Perhaps Edom was also being destroyed, or maybe they were asking about the state of their Babylonian allies. Either way, God's watchman says, "the morning is coming"(v.12), which means that judgement is soon over. But the watchman also says that ''night will come again"(v.12), which meant that more destruction is still ahead. Therefore God's good news for Israel, "Babylon has fallen"(v. 9), is God's bad news for all of their enemies.