2 Kings 8.25-[etc.]
[Summary: Ahaziah King of Judah and Joram the King of Israel are both descendants (Ahaziah through his mother) of Omri, Ahab's father. Both went to war together against Hazael King of Syria.]
And King Joram returned to be healed in Jezreel of the wounds which the Syrians had given him... and Ahaziah... went down to see [him]... because he was sick.
Then Elisha the prophet called one of the sons of the prophets and said to him, "Gird up your loins, and take this flask of oil in your hand, and go to Ramoth-gilead. And when you arrive, look there for Jehu the son of Jehoshaphat, son of Nimshi; and go in and bid him rise from among his fellows, and lead him to an inner chamber.
"Then take the flask of oil, and pour it on his head, and say, 'Thus says the Lord, I anoint you King over Israel.' Then open the door and flee; do not tarry!"
So the young man, the prophet, went to Ramoth-gilead. And when he came, behold, the commanders of the army were in council; and he said, "I have an errand to you, O commander."
And Jehu said, "To which of us all?"
And he said, "To you, O commander."
So he arose, and went into the house; and the young man poured oil on his head, saying, "Thus says the Lord the God of Israel, I anoint you King over the people of the Lord, over Israel. And you shall strike down the house of Ahab your master, that I may avenge on Jezebel the blood of my servants the prophets, and the blood of all the servants of the Lord. For the whole house of Ahab shall perish; and I will cut off from Ahab every male, bond or free, In Israel. And I will make the house of Ahab like the house of Jeoboam the son of Nebat, and like the house of Baasha the son of Ahijah. And the dogs shall eat Jezebel in the teritory of Jezreel; and none shall bury her."
Then he opened the door, and fled.
When Jehu came out to the servants of his master, they said to him, "Is all well? Why did this mad fellow come to you?"
And he said, "You know this mad fellow, and his talk."
And they said, "That is not true; tell us now."
And he said, "Thus and so he spoke to me, saying 'Thus says the Lord, I anoint you King over Israel.'"
Then in haste every man of them took his garment, and put it under him on the bare steps; and they blew the trumpet, and proclaimed, "Jehu is King!"
Thus Jehu the son of Jehoshaphat the son of Nimshi conspired against Joram. [recap]... So Jehu said, "If this is your mind, then let no one slip out of the city to go and tell the news in Jezreel." Then Jehu mounted his chariot, and went to Jezreel, for Joram lay there. And Ahaziah King of Judah had come down to visit Joram.
Now the watchman was standing on the tower in Jezreel, and he spied the company of Jehu as he came, and said, "I see a company."
And Joram said, "Take a horseman, and send to meet them, and let him say, 'Is it peace?'"
So a man on horseback went to meet him, and said, "Thus says the King, 'Is it peace?'"
And Jehu said, "What have you to do with peace? Turn round and ride behind me."
And the watchman reported, saying, "The messenger reached them, but he is not coming back." Then he sent out a second horseman, who came to them and said, "Thus the King has said, 'Is it peace?'"
And Jehu answered, "What have you to do with peace? Turn round and ride behind me."
Again the watchman reported, "He reached them, but he is not coming back. And the driving is like the driving of Jehu the son of Nimshi, for he drives furiously."
Joram said, "Make ready." And they made ready his chariot. Then Joram King of Israel and Ahaziah King of Judah set out, each in his chariot, and went to meet Jehu, and met him at the property of Naboth the Jezreelite.
And when Joram saw Jehu, he said, "Is it peace, Jehu?"
He answered, "What peace can there be, so long as the harlotries and sorceries of your mother Jezebel are so many?"
Then Joram reined about and fled, saying to Ahaziah, "Treachery, O Ahaziah!"
And Jehu drew his bow with his full strength, and shot Joram between the shoulders, so that the arrow pierced his heart, and he sank in his chariot.
Jehu said to Bidkar his aide, "Take him up, and cast him on the plot of ground belonging to Naboth the Jezreelite; for remember, when you and I rode side by side behind Ahab his father, how the Lord uttered this oracle against him: 'As surely as I saw yesterday the blood of Naboth and the blood of his sons-- says the Lord-- I will requite you on this plot of ground.' Now therefore take him up and cast him on the plot of ground, in accordance with the word of the Lord."
When Ahaziah the King of Judah saw this, he fled in the direction of Beth-haggan.
And Jehu pursued him, and said, "Shoot him also." And they shot him in the chariot at the ascent of Gur, which is by Ibleam. And he fled to Megiddo, and died there.
[After this we have a pretty thorough massacre, starting with Jezebel & including all of Ahab's friends & relatives. You had to be there to appreciate it... Ahaziah's mother seizes power in Jerusalem, over Judah, and rules until there's a coup in favor of his last surviving son, who has been hidden away for the six years of her reign.]