Knight-errant

He made the decision to become a knight-errant.

Source: tatoeba (7799907)

In short, his wits being quite gone, [Don Quixote] hit upon the strangest notion that ever madman in this world hit upon, and that was that he fancied it was right and requisite, as well for the support of his own honour as for the service of his country, that he should make a knight-errant of himself, roaming the world over in full armour and on horseback in quest of adventures, and putting in practice himself all that he had read of as being the usual practices of knights-errant; righting every kind of wrong, and exposing himself to peril and danger from which, in the issue, he was to reap eternal renown and fame.

Source: wiktionary

Before Tarzan had returned to the tribe, a certain young bull, not being able to secure a mate from among his own people, had, according to custom, fared forth through the wild jungle, like some knight-errant of old, to win a fair lady from some neighboring community.

Source: wiktionary

"Welcome, my young knight-errant!" exclaimed Madame de Soissons. "I am expecting you to do wonders." "Nothing could be wonderful when performed in your service," replied the boy, with that readiness of compliment so characteristic of his time and court.

Source: wiktionary