Alien Missions (Hardmode)
The alien forces will dispatch UFO sorties to conduct an array of hostile acts or missions across the Earth, X-COM can respond in kind by deploying its own interceptors and assault transport ships to interdict these missions and disrupt the aliens operations.
The ultimate objective of any alien missions is to generate a negative score against X-COM, or to setup for future missions that will further generate more severe negative score events.
Mission Generation
At the commencement of any month, the aliens will schedule a random ordinary mission and a terror mission to occur. In the latter stages of the game an additional ordinary mission will be scheduled.
An exception to this is in the very first month of the game the aliens are guaranteed to schedule an alien base construction mission within the same region as X-COM's starting base, this is to give X-COM players the chance to conduct an alien base assault to advance the tech tree, and to avoid a situation where the player must wait for an extended period of time for the aliens to construct a base.
If there is an alien base present on Earth, there is a 6% chance every day that an alien supply mission will be scheduled.
If X-COM intercept any UFO in the air, there is percent chance (20% on superhuman) for an alien retaliation mission to be immediately scheduled.
Mission Types
Alien Research
A relativity benign mission, the aliens conduct basic research work across a target zone, typically landing in remote and isolated regions and attempting to avoid contact with humans. A sortie of a small and medium scout, followed by a excavator, are deployed for these missions. The aliens only score 10 points if the mission is accomplished unopposed.
Alien Harvest
The aliens harvest local Earth fauna for unknown reasons, stealing organs and other biological matter. While a complicated operation, this mission draws only minimal concern from world governments. The aliens will deploy all forms of scouts, fighter and sentry UFOs, harvesters and a lab ship. Completion of this mission will score 30 points against X-COM.
Alien Abduction
An insidious operation, the aliens abduct and perform unknown experiments on humans who are captured from isolated locations, not all humans make it back. The alien deploy a wider range of UFOs for these missions, including small and medium scouts, an excavator, lab ship and abductor UFO. Completion of this mission confers negative 50 points.
Alien Infiltration
A highly complicated mission intended by the aliens to force a council nation to withdraw all political and economic support from X-COM. A small task force of scouts and a fighter UFO will lay the initial groundwork by conducting reconnaissance sorties, once completed a large contingent of UFOs will rapidly arrive within the target country, starting with a sentry UFO and ending with a battleship. Upon completion the target nation will withdraw from the X-COM project, confer a negative 1,500 score at the end of the month, and see the construction of an alien base within the nation's borders.
Destruction or interception of the UFO's attending to this mission will only delay its outcome, it cannot be prevented once the aliens commit to it.
Alien Base Construction
Alien forces attempt to construct a ground base somewhere on Earth. Initial waves of UFO's consist of scouts and an excavator who will land to conduct survey and construction work, a final sortie a supply, lab and a battleship will arrive within a short window to inaugurate the completion of the base construction while also landing to offload supplies and personal. Completion of this mission deducts 50 points of score, with the base itself deducting 5 points in its region per day.
Alien Supply Mission
So long as an alien base remains operational, it has a 6% chance per day to request a supply mission at midnight. A supply mission sees a single alien supply ship deployed directly to the base to resupply it, it will remain landed for a while before leaving. The aliens score 30 points if the mission is a success.
Alien Terror Mission
The aliens will conduct reconnaissance work to pinpoint a target site for a terror mission. A fighter and sentry UFO will conduct the initial reconnaissance before a terror ship assists in preparation work. If the recon is completed a final terror ship will be launched with the terror forces onboard, and will head straight for the target city. Interception of the final terror ship will prevent the terror attack. Completion of this mission deducts 20 points of score, in addition to the ~1000 to ~1500 score deduction if the terror mission is a success for the aliens.
Retaliation
The most overtly aggressive of the Alien Missions, Alien Retaliation signifies an alien attempt to destroy X-Com via military force. Alien Retaliation missions are generated in response to X-Com's activities either by X-COM interception flying UFOs or by the war dragging on long enough. Retaliation missions come in two phases, first a series of UFOs will conduct scouting operations in the region the aliens suspect an X-COM base is located, this will include a small scout, fighter ship, sentry ship and a battleship. The scouts never land, and must be intercepted by aircraft to hinder them.
If the scouts locate an X-COM base, a final battleship will be deployed and streak directly towards the X-COM base to assault it. If the scouts are destroyed then the aliens may fail in finding the base. The aliens do not directly score from this mission, but the risk of the loss of a base represents a major blow to X-COM.