This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Digimon Digital Card Battle
The ending gets really dark. After defeating VenomMyotismon he starts repeating the same lines over and over. Then the game starts being hacked by a man known as A which causes the screen to go black and later all the colors change. At this part of the game, the player regains control but can neither leave the town nor save progress making this reminiscent of Survival Horror.
Let's not forget A's VenomMyotismon avatar which looks creepier than ever.