Alien Terror
When the Aliens terrorise a city, they will deploy some special forces with awesome powers. Civilians will be directly threatened, and governments will be forced to evacuate whole areas. The main purpose behind this activity is to generate sufficient public hysteria so that governments will threaten the X-COM project.
If successful, the aliens will score 1000 points, which is eventually subtracted from your score. However, merely arriving at the Terror Site and aborting immediately will render the Terror Mission a failure for both parties. The aliens will only gain points equal to the number of killed civilians.
Usually aliens send a ship to scout the area for a city in preparation for a terror attack. Shooting down or assaulting the craft may either interrupt or delay the arrival of the terror ship. Once the terror ship arrives, it will head toward the targeted city and conduct a terror attack there, unless it is shot down.
