Basic Trope: A Cosmic Force makes sure that good and evil must always be equal.
Straight: The Good White Knight slays the Evil Black Sorcerer. As a result the Cosmic Force spawns an Evil Knight, a new Black Sorcerer, and the Doom Dragon to balance the sudden spike of good.
When the Good White Knight slays the Evil Black Sorcerer, the Cosmic Force spawns an entire army of Evil Knights and Doom Dragons who follow the Sorcerer of the Black Hole, all to balance the sudden spike of good.
A balance must be kept between balance and imbalance.
The good White Knight slays the Evil Black Sorcerer. The Sorcerer's apprentice takes his place after his death, making the balance equal quietly.
The good White Knight slays the Evil Black Sorcerer. After 10 years of peace, another one comes up in some form to fill the power void.
Justified: When the Good White Knight kills the Evil Black Sorcerer, his right-hand man, inspired by the Cosmic Force, regroups elsewhere and becomes the new Big Bad, finding that he is at least as good at creating Doom Dragons as the Sorcerer was.
Inverted: The Good White Knight kills the Evil Black Sorcerer, which is followed by the rise of the Magician of Light.
Subverted: The Good White Knight prepares to face the Sorcerer of the Black Hole, only to find that it was a friend in disguise, playing a practical joke on him.
Double Subverted: The Good White Knight prepares to face the Sorcerer of the Black Hole, only to find that it was a friend in disguise, playing a practical joke on him. However, it turns out that the friend really was the Sorcerer of the Black Hole using magic to conceal himself, hoping to lull the Knight into a sense of complacency.
The Cosmic Forces only have Pathetic Goblins available to balance good and evil after the Sorcerer dies... so they send in 999,999,999 of them to make sure that all is balanced.
A balance must be kept between Balance and Imbalance.
The Evil of the imbalance set up by a sudden influx of Good itself restores the Balance Between Good and Evil. This means the Evil is no longer there, and the world is in Imbalance, meaning the Evil is there and the world is Balanced, meaning...
Evil and Good have literal spawn points, and any time one or the other is defeated a new one appears instantly.
Zig Zagged: ???
When Good wins, there is no Cosmic Force creating more Evil to balance the scales.
Good and evil don't have any cosmic significance.
Evil is treated as an unbalancing force and is eradicated with extreme prejudice.
The writers are trying to explain why villains continue appearing no matter how many the heroes defeat.
The Good White Knight, finding that evil continues to rise up no matter what he does, becomes bitter and gives up, refusing to fight the never-ending evil if he can't make a difference in the world.
Evil and Good are forced to coexist, which means they're in almost constant conflict. After thousands of years of this, the land the work takes place in is a miserable, ruined, wasteland, with disgusted and weary heroes battling it out with equally disgusted and weary villains. Neither can wipe the other out, no one can actually accomplish anything constructive, and everyone's pretty unhappy.
Reconstructed: However, the Shining Squire, unable to get his master the Good White Knight to change his mind, takes on his master's mantle and fights evil in his place, reasoning that even if more evil rises up against him, at least he had done what was right.