Where did the messenger go to tell Jezebel what Elijah had done?
The messenger traveled from Mount Carmel across the valley to Jezreel, identified as the second government palace, to report Elijah's actions to Jezebel. There Jezebel heard what had happened and responded with a threat against Elijah. This was the destination of the messenger.
After Elijah's confrontation with the prophets of Baal on Mount Carmel, where fire from Jehovah consumed the sacrifice and the water, Elijah demanded that the 400 prophets of Baal be killed. A messenger then left Mount Carmel, crossed the valley, and went to Jezreel, which was their second government palace, to tell Jezebel what had happened. Jezebel responded by sending word back that by the next day Elijah would be as dead as her prophets. The context notes Israel's tendency to sink into unbelief despite all the manifestations of Jehovah's power.