Athaliah (Hebrew Atalyahu, "Yahweh is exalted") was the queen of Judah during the reign of King Jehoram, and later became sole ruler of Judah for five years. Albright has dated her reign to 842 - 837 BC, while Thiele offers the dates 841 - 835 BC. She was the daughter of King Ahab of Israel and her marriage to Jehoram sealed a treaty between Israel and Judah.
Jehoram of Judah, a descendant of King David, actively promoted the worship of Yahweh in his country, but he tolerated Athaliah's worship of Baal. After Jehoram's death, their son Ahaziah became Judah's king with Athaliah acting as queen mother. She used her power in that role to establish the worship of Baal in Judah after Ahaziah was killed in a state visit to Israel along with the then-king of Israel, also named Jehoram, who was Athaliah's brother. Jehu assassinated them both in Yahweh's name and had Athaliah's entire extended family in Israel murdered.
Athaliah, as queen of Judah, had all possible successors to David executed except one. However, a grandson of hers named Joash escaped the purge and was raised in secret by the priest Jehoiada. Six years later, Athaliah was surprised when Jehoiada revealed Joash and proclaimed him king of Judah. She rushed to stop this rebellion, but was captured and executed.
The story of her actions is told in 2 Kings 8:25 - 11:16.