Terror Mission (TFTD)

While the aliens are usually very passive in their dealings with the human populace in general, continual interference by X-COM will lead them to attack a populated area in order to provoke X-COM to action, or preferably inaction in an effort to make them look bad in the eyes of the Council of Funding Nations, thus reduce their standing and hopefully force countries to withdraw their funding.

Traditionally, the aliens in the first alien war attacked small country towns with a low population. In the second alien war, the scope has broadened. As the new alien threat are nautical in nature, so are their targets. Terror sites now occur in small ports, island resorts and ships.

All of these are conducted on land. Though this eliminates the ability of the aliens to use their devastating torpedoes, they bring deadly terror units into battle to compensate.

There is also the Artefact Sites, where the aliens are activating synonium devices in old temples on the sea bed. Though not strictly terror missions as no civilians are involved, they have the same urgency as terror sites due to the short time frame available to you to attend to them. If they are not dealt with before the devices are restored, the blow to X-COM's standing with the Council of Funding Nations can be just as if not more disastrous than failing to attend to a terror site.

Artefact sites and ship missions are classified as two-parter missions. You must go through two maps in succession in order to clear the site. They are lengthy missions so you will need to plan your ammunition supplies accordingly.

Port and island missions are the closest to a traditional terror site that you can get and you only need to clear the infestation of aliens on the map.

For more information on the different terror mission types, refer to their respective articles.

Timing

Terror missions almost always occur on the first of the month, so make sure your troops are prepared for action at that time. Also, they appear during the dead of night in whichever location they have chosen and will disappear before dawn, seemingly guaranteeing that you will have to endure the additional hardship of a night mission.

However, there does seem to be a trick to get around this. Terror sites will not disappear as long as they are targeted by an X-Com craft. A couple hours before dawn approaches the terror site, send your craft in the opposite direction from the site. As dawn approaches, reroute the craft towards the terror site. The extra distance the craft needs to cover will make it so that you arrive during daylight hours, making what is normally a difficult and grueling mission a little bit easier.