A specific form of Split Personality. Maybe the Body Horror became a bit too fused with someone. Maybe the Unstoppable Rage is getting... too unstoppable. Perhaps The Atoner's past is taking on a life of its own. A half-human's Demon / Vampire self is trying to take control, or that Deal with the Devil is hitting a bit too close to home.
Either way, the enemy is behind the hero's eyes, and its time is coming when it can take over. Until then, it'll do all it can to control him and get him to give in to its Horror Hunger. The thing to stress most is that the Enemy Within is the hero. He or she cannot simply exorcise it out. Often the Enemy Within is the cause of the powers that the hero has that allows them to do what they do. With Great Power Comes Great Insanity, remember?
Often, since Evil Is Cool and Evil Feels Good, other characters may realize the danger before the hero and need to convince him.
This trope has received criticism from the mental health community, as it is accused of demonising patients with Dissociative Identity Disorder (or DID), and being seen as one of the reasons for stigmatization against people with DID due to its constant use in media. This was most notably seen in the backlash against Split (2016), whose potrayal of DID was seen by many to be VERY damaging.
Not to be confused with the expansion pack to the X-Com reboot, nor an unrelated TV series, or another unrelated novel. Contrast Enemy Without. Compare Jekyll & Hyde. Also not to be confused with Kill It Through Its Stomach, which involves an enemy literally inside another. Usually part of their Soul Anatomy.
Enemy Within has been suggested to play 2 roles. Click below to see other actors suggested for each role, and vote for who you think would play the role best.
