There is a source of innocence - whether it be a person or creature of Incorruptible Pure Pureness or just a MacGuffin that represents purity. Either way, destroying it has the potential to grant great power.

This leads to a conflict in which the heroes must protect said person, creature, or object in the hopes of keeping it safe from the villains who want it for gain. In some other cases, the heroes might consider this - just in time to finally back away once they realize what they're doing.

Only applies if the power is granted specifically because of the innocence. Bonus points if the great power granted has some kind of curse attached. This is a staple of The Undead, zombies, necromancers and vampires in general. When destroying one's own innocence, may count as Power at a Price. Depending on how it's played, this could be considered the villain's Moral Event Horizon...if they haven't already crossed it. Powered by a Forsaken Child has shades of this.

Anime And Manga

• Inuyasha has the Shikon gem. It's totally pure after it comes out of Midoriko, but gets defiled before getting handed off to Kikyo. Then Naraku tries to taint it by sabotaging her relationship (among other plots).
• In Puella Magi Madoka Magica, one of the best energy sources in the universe is in the refined suffering of preteen human girls.

Literature

• Drinking unicorn blood is portrayed this way in Harry Potter; it allows one to gain strength, but it curses the drinker to a half-body for the crime of slaughtering the unicorn. Quirrell drinks it to support Voldemort on the back of his head.
• In Percy Jackson and the Olympians, there is a prophecy that whoever kills the Ophiotaurus ("Bessie") and sacrifices its entrails to fire will gain the power to topple the gods, as there is power in killing innocence. Thalia is briefly tempted with it. Percy elects to have Bessie protected in Olympus inside a giant aquarium instead of having the gods destroy it to prevent anyone from taking up that power. Thankfully, nobody acts on it in the course of the series, though it is yet to be revealed whether it will occur in The Heroes of Olympus.

Live-Action TV

• Xena: Warrior Princess: Xena and Gabrielle meet up with some followers of the One God. X&G (and we) think that they're following the God of the Israelites, but it turns out they actually follow the evil god Dahok. In order for Dahok to manifest on Earth it needs the sacrifice of an Innocent. Gabrielle thinks that she stops the sacrifice by preventing them from killing her friend on the altar, but what they were really after, and got, was Gabrielle's blood-innocence: in order to stop the alleged sacrifice she had to kill the person who was about to kill her friend.

Tabletop Games

• A terratomorph in Dungeons & Dragons is a creature of chaos that feeds by destroying order - leaving a trail of randomly warped landscape in its wake. The terratomorph itself is a barn-sized amoeba in wildly shifting colors.

Web Comics

• During the time-travel arc of Looking for Group, Cale is forced to kill an innocent child in order to protect a great city, because the city's shields are Powered by a Forsaken Child, and by killing him, they could pull the entire city forwards in time to when they needed it... Only not. The child was actually a fake, but the trope still applies - the sacrifice that was actually needed was Cale's innocence, in being forced to darken his hands with the blood of an innocent.
• In 8-Bit Theater, obtaining the Hadoken required the sacrifice of a dozen or so children. This is played for Comedic Sociopathy.
  • After obtaining the Hadoken, in order to continue using it, it needs to be powered by love. Meaning every time it is used it destroys some of the love in the universe making it a colder, darker place.