In 2 Kings 21:1-18, Manasseh is portrayed as a purely wicked king of Judah. In 2 Chronicles 33, Manasseh is also described as a bad king but with the added detail of his repentance.
Read my post on What is the Difference Between Samuel/Kings and Chronicles?
Wouldn’t it be great to have more information about that repentance?
That is what we get (sort of) in the apocryphal book of The Prayer of Manasseh. This not the actual prayer that Manasseh prayed, but rather a later writer’s attempt to reconstruct what such a prayer could have looked like. This prayer was likely written in the second or first century BC.
You can find the text for The Prayer of Manasseh online but I’m using the one found in the The New Oxford Annotated Bible.
If you want to learn more about the Apocrypha, I recommend that you listen to this podcast episode.