12. The Alien Force In Day Of The Dove
A life form that feeds on conflict and hatred fuels a battle between the Enterprise crew and the stranded crew of a Klingon battle cruiser.
It warps their perception of reality and gives them false memories. The most notable instance of how the creature can alter memories is Chekov's insistence he lost a brother to a Klingon raid when in reality, he is an only child.
This mysterious being even has the power to revive the dead. Neither side can truly know if what they are seeing is real or an illusion fed to them by the creature. It's only when both sides realize they are being used that they come up with a plan to drive the creature off the Enterprise.