This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Halo: Reach
Look closely at the background during the mission where you're flying around New Alexandria's skyscrapers rescuing people and destroying signal jammers. While you're desperately trying to reestablish a link to command and save the few people remaining, the Covenant is systematically glassing the entire city around you. Every couple of minutes the ships overhead fire their main guns, and more and more of the city is reduced to melted, burning ruins. By the end of the mission almost the entire horizon is glowing a fierce red. In the next mission, and for the rest of the game, the once-beautiful, vibrant planet is an ashen hellscape utterly devoid of living plants or animal life.
The Covenant. The Grunts are still cowardly who run, the Brutes are still savages, the Elites are still sword-wielding samurai aliens, and the Jackals are still moving cover, but there's something about the redesign with the Covenant species that makes you seem...puny. It's almost like you're finally realizing that the Covenant are more than just bumbling comical armies. Hell, the Elite that roars at Six in WINTER CONTINGENCY; the Elites did not have those menacing teeth at Alpha Halo or the Ark!