Manasseh, also spelled Manasses, king of Judah (reigned c. 686 to 642 BCE). During his long and peaceful reign, Judah was a submissive ally of Assyria. In the course of his reign there occurred a revival of pagan rites, including astral cults in the very forecourts of the temple of Yahweh, child sacrifice, and temple prostitution; hence, he is usually portrayed as the most wicked of the kings of Judah.
If he had any tendencies toward independence from Assyrian domination, they apparently were suppressed by his being taken in chains to Babylon, where he was molded into proper vassal behaviour, although one edifying and probably unhistorical biblical account reports his repentance and attempt at religious reform after his return to Judah.
He engages in all manner of prohibited religious practices and basically winds up being the last straw, as far as God is concerned. The disobedience of Manasseh provokes God into finally ordering the people of Judah into exile, allowing the Babylonians to invade and destroy.
Even though the righteous king Hezekiah is his father, Manasseh becomes wicked—following a typical pattern where latter kings, whether Israelite or non-Israelite, tend to be worse than their predecessors (the Babylonian Belshezzar and the Greek Antioch Epiphanes both being good examples of decline).
He reigns a record-setting 55 years in Judah, does abominable things like sacrificing one of his children in fire, and practices divination and sorcery, before he eventually dies. Appropriately enough—given that he makes Judah forget God—his name means “causing to forget.”