Ahab, the king of Israel, was a follower of Baal, which, of course, Yahweh and Elijah didn't like very much. So Elijah suggested a prayer contest to determine which god was the real God.
Elijah told Ahab to get all the prophets of Baal to meet him on Mt. Carmel.
Send, and gather to me all Israel unto mount Carmel, and the prophets of Baal four hundred and fifty, and the prophets of the groves four hundred, which eat at Jezebel's table. 1 Kings 18:19
When they arrived, he told them to get a bull, kill it, chop it into pieces, and put the whole bloody mess on some wood. But don't light it on fire.
Let them therefore give us two bullocks; and let them choose one bullock for themselves, and cut it in pieces, and lay it on wood, and put no fire under: and I will dress the other bullock, and lay it on wood, and put no fire under. 18:23
Then ask Baal to light it all on fire.
And call ye on the name of your gods, and I will call on the name of the LORD: and the God that answereth by fire, let him be God. 18:24
So the prophets of Baal did that. They prayed and prayed all morning and jumped up and down on the altar, but nothing happened.
They took the bullock which was given them, and they dressed it, and called on the name of Baal from morning even until noon, saying, O Baal, hear us. But there was no voice, nor any that answered. And they leaped upon the altar which was made. 18:26
Elijah taunted them, saying, "Pray harder! Maybe your god is on a trip or sleeping or something." But still there was no fire.
And it came to pass at noon, that Elijah mocked them, and said, Cry aloud: for he is a god; either he is talking, or he is pursuing, or he is in a journey, or peradventure he sleepeth, and must be awaked. 18:27
The priests of Baal cut themselves with knives until their blood was gushing around all over. But nothing happened.
And they cried aloud, and cut themselves after their manner with knives and lancets, till the blood gushed out upon them ... until the time of the offering of the evening sacrifice ... there was neither voice, nor any to answer, nor any that regarded. 18:28-29
Then it was Elijah's turn. Just to show off, he poured water over the whole bloody mess three times.
He put the wood in order, and cut the bullock in pieces, and laid him on the wood, and said, Fill four barrels with water, and pour it on the burnt sacrifice, and on the wood. And he said, Do it the second time. And they did it the second time. And he said, Do it the third time. And they did it the third time. And the water ran round about the altar; and he filled the trench also with water. 18:33-35
He prayed to God and a fire fell down from heaven and consumed it all, even the surrounding stones.
The fire of the LORD fell, and consumed the burnt sacrifice, and the wood, and the stones, and the dust, and licked up the water that was in the trench. 18:38
Then Elijah told the spectators to slaughter the 450 prophets of Baal (along with the 400 prophets of the grove? See 18:19).
Baal's prophets are four hundred and fifty men ... And Elijah said unto them, Take the prophets of Baal; let not one of them escape. 18:22, 40a
So that's what they did.
And they took them: and Elijah brought them down to the brook Kishon, and slew them there. 18:40b
And the hand of of the Lord was on Elijah.
And the hand of the LORD was on Elijah. 18:46