What is the literary term for the redemption act of a character?

Solution 1:

If a character makes up for past bad actions by some outstanding effort, redemption, "the act of redeeming" or "salvation from sin" probably is among the most suitable terms. Repentance, "A feeling of regret or remorse for doing wrong or sinning" is a related term.

Absolution, suggested by Irene, is of similar tone: "An absolving, or setting free from guilt, sin, or penalty; forgiveness of an offense", but of course involves another agent, while one can repent or redeem oneself without aid.

Even within fiction, thoroughgoing changes such as redemption or absolution of a bad person may be hard to believe. To avoid implausibility, authors usually settle for lesser breakthroughs among their characters. A character may have an epiphany, "An illuminating realization or discovery, often resulting in a personal feeling of elation, awe, or wonder" during a story's dénouement. The Characterization article in wikipedia notes:

In a tragedy, the central character generally remains fixed with whatever character flaw (hamartia) seals his fate; in a comedy the central characters typically undergo some kind of epiphany (sudden realisation) whereupon they adjust their prior beliefs and practices and avert a tragic fate.

Dénouement is a term for the portion of the story within which such character changes as you ask about occur. wiktionary cites Henrik Ibsen:

In a ‘well-made’ play, action should be organised in three sections: exposition of the central problem, alarms and excursions, dénouement. The plot should hinge on a secret or a dilemma which affects the main character; the audience is allowed only hints and glimpses of this as the play proceeds, and all is fully revealed only as the action moves towards dénouement.

On the same topic, wikipedia comments:

The dénouement... comprises events between the falling action and the actual ending scene of the drama or narrative and thus serves as the conclusion of the story. Conflicts are resolved, creating normality for the characters and a sense of catharsis, or release of tension and anxiety, for the reader. ... Exemplary of a comic dénouement is the final scene of Shakespeare’s comedy As You Like It, in which couples marry, an evildoer repents, two disguised characters are revealed for all to see, and a ruler is restored to power. ...

In short, some of the relevant literary terms are epiphany, falling action, dénouement, and catharsis.

Solution 2:

By killing the Emperor and saving Luke, Vader has made atonement.

Solution 3:

Another term you can use for this situation is absolution.

Solution 4:

The verb form for redemption is pretty straightforrwad. Instead of

he then undergoes _

it should rather be

he then redeems himself