Transmigration trope

Transmigration is a Chinese trope popular in webnovels and also seen in C-drama. The story generally follows a protagonist who died, and then transmigrates into a new world, into an existing persons body. The new world is often from a book the character read, and they use the knowledge from their past life to achieve their goals.

Some transmigration series include a system, which is a game-like mechanic that sets tasks or quests for the person who transmigrated, usually to ensure certain plot events occur. Rewards may be given for successes, or the system may decide to mete out punishment for failures, including the threat of sending the character (typically called a 'user') back to 'their own world'. 

Since the user is often dead in their world, it forces them to abide by the System's demands, even if the task is something that may get them in trouble or even killed. Often, the type of character a user transmigrates into is a villain with a bad reputation, usually destined to have a horrible death.

FMC = Female Transmigrator
MMC = Male Transmigrator