The ghost is a NPC blueprint that targets and deals damage to players. The ghost has no physical configurable aspects and cannot be connected to logic. Ghost have no reaction to player fire, which means they cannot be killed. Also, the Ghost has range boundaries like the turrets, which means if players escape its boundaries it will stop chasing them. When a Ghost comes into contact with players, it will drain their health; if a player is not moving and the Ghost is draining his/her health, he/she will die in under 10 secs.
In the past the ghost was not for free and you needed to buy it.