It's one of the standard themes of Campbell's Hero's Path. The would-be hero descends into the Underworld (or any other unfamiliar situation [that which is not]), usually nearly dying and being reborn as the hero, faces the dragon/minotaur and defeats them, takes the treasure and returns to the real world where this change is lauded/rewarded (or at least remarked upon). In many cases they are (mythologically at least) not the same person anymore.

One of the things about that irritates me about RPGs that use this theme is that after killing the dragon and taking the treasure, they go look for another dragon to slay and another treasure to steal; there is never really any Return for the players/characters to measure how they have changed. Thus, while they may grow in power, they never really change or grow.