UFO Incursions (Apocalypse)

This page lists the various nuts and bolts of how the game decides which Alien Craft come out of the Dimension Gates and what they do once in Mega-Primus.

Unlike in UFO: Enemy Unknown and X-Com: Terror From the Deep, the Aliens in X-COM: Apocalypse do not have infinite reserves of craft. Rather, the pool of craft in the Alien Dimension is tracked, with new UFOs being constructed every Monday (up to a limit) and X-COM interceptions removing craft from the pool.

Every time an alien mission is scheduled, the game rolls randomly on a table to decide what act they will attempt to perform. This table changes every week. Then, once an act has been selected, the specific UFO makeup of the incursion is chosen deterministically from tables for each possible act depending on what craft are available in the Alien Dimension pool; as reproduced here, the tables increase in priority downward, so the aliens will always use the highest-numbered fleet possible for the chosen mission.

Some UFOs on the Infiltration and Overspawn Drop tables are listed as "Bombing" craft. These craft will bomb a building, as though on an Air Raid mission.

Destroying the Control Chamber building permanently halts the production of alien craft. It also triggers a special incursion not found on these tables - the next time an alien mission is scheduled, all remaining UFOs (except Probes) will swarm out of the Dimension Gates and start tearing down Mega-Primus as though on a very large Bombing mission. This final battle is typically referred to as "the Apocalypse mission".

UFO Production

Week
1	9	9
2	2	4	3	2
3	2	4	2	2		3
4	1	2	1	1	2	2
5		1		1	1	1	2
6		1		1	1		1	1
7		1			1		1	1	1
8		1				1	1	1		1
9								1	1
10		1					1
11				1		1			1	1
12							1	1
13		1		1					1
14								1		1
15									1
16+							1	1	1	1
Maximum	15	15	6	6	6	6	6	6	4	2

Mission Chance

Week	Infiltration	Base Assault	Bombing	Overspawn Drop
1-3	100%	0%	0%	0%
4	90%	0%	10%	0%
5-6	80%	10%	10%	0%
7	70%	10%	10%	10%*
8	50%	20%	10%	20%
9-11	40%	20%	20%	20%
12	30%	30%	20%	20%
13	30%	20%	30%	20%
14	40%	20%	20%	20%
15-18	30%	20%	20%	30%
19	20%	20%	30%	30%
20+	10%	40%	20%	30%

*An Overspawn drop in Week 7 can't actually happen, as there are guaranteed to be no Motherships available (none have spawned yet).

Infiltration Fleets

These are the standard type of UFO incursion for the majority of the early game, which peters out as the weeks go on in favor of more direct aggression. Infiltration fleets consist of one to three primary craft, usually with escorts to protect them (as seen on the table below). When they appear in Mega-Primus, the primary craft will fly directly to a pre-selected target destination, hover over it for several seconds and attempt to deploy a squad of aliens to infiltrate the building via a transport beam (i.e. 'the grey tube'). Destroying the craft before it reaches its destination will prevent this. If the tube has already appeared, it is possible to prevent the UFO from succeessfully completing its transport by destroying it before the beam has finished. UFOs which successfully deposit their troops will then immediately move to the dimension gates to return to the Alien Dimension, leaving players to have to send squads to investigate those buildings for aliens. If left unchecked, the target organization's infiltration percentage will go up after several hours and the aliens may continue to spread to nearby buildings and organizations, creating an even bigger problem for X-Com over time. When a primary UFO craft for this mission type escapes via the dimension gates, that craft's assigned escort(s) will then immediately attempt to escape as well (this rule holds true for most if not all incursions).

Mission	Primary	Escort	Bombing	Week
1				1
2				1
3				2
4				2
5				2
6				4
7				3
8*				4
9				3
10				3
11				4
12				4
13				5
14				8
15				8
16*				7
17**				7
18				7
19				8
20				8

*These fleets will never appear, as the ships required will also fulfil a higher-priority fleet.

**This fleet can appear as long as the aliens have at least one Battleship and one Escort Ship (though two Battleships will still appear in Mega-Primus). This is the result of a bug in the spawn routine; instead of checking whether there are two Battleships, it checks whether there is one Battleship twice.

• UFO fleets led by Transporters or Assault Ships are always on Infiltration missions.

Bombing Fleets

Likely the second or type of fleet the player will have to contend with (or third depending on luck), 'Bombing' fleets are exactly that - UFOs sent to cause as much destruction to the city, craft and X-Com as possible. These typically consist of primary craft using disruptor bombs or multi-warhead missiles while escorted by Fast Attack or stronger escort ships using medium or larger disruptor beams. On the plus side, players will not have to worry about tactical missions other than to seize any downed UFOs, as they will not be attempting to deposit troops in buildings. These fleets will remain in the city much longer than infiltration fleets before pulling back to the alien dimension, and can cause massive damage if left unchecked. While they can be a great source of captured UFOs and technology, a bombing fleet should never be underestimated, as it can pose one of the greatest threats to unprepared X-Com craft and personnel. If an X-Com base tile containing the 'person' icon is destroyed, that base's personnel may be killed as a result.

Mission	Primary	Escort	Week
1			4
2			4
3			5
4			4
5			5
6			6
7			7
8			9
9			9
10			8

• Bombing fleets appear from Week 4 onwards.

Base Assault Fleets

This fleet type is the reason players want to build Security Stations and keep armed personnel to secure X-Com facilities. Base Assault Fleets will spend their time in the city scanning buildings with a sparkling blue scanning beam resembling blue 'rain' with the express purpose of seeking out X-Com facilities. Once located, the aliens will send at least one UFO in all future missions of this type loaded with troops with one singular purpose: Invading and destroying the discovered base. This can be prevented by shooting down any incoming UFOs before they can locate your base, or the troop transport if they've already found it (though it can be an impossible shell game to determine just which primary UFO is the one bringing the invaders). There is an urban legend that this beam can cause an organisation to be immediately infiltrated, leading to the infamous misnomer 'micronoid rain' for the beam.

Mission	Primary	Escort	Week
1			5
2			5
3			5
4			5
5			5
6			5
7			5
8			5
9			7
10			8

• Base Assault fleets can appear from Week 5 onwards.

Overspawn Drop Fleets

Exactly what it sounds: An Alien Mothership (usually with escort craft) is sent to drop an Overspawn to attack the city directly. This type of mission can *only* occur if the aliens currently have a mothership available to deploy.

Mission	Primary	Escort	Bombing	Week
1				8
2				8
3				8
4				8
5				11

• Overspawn drops can appear from Week 8 onwards.
• A fleet with two Motherships is guaranteed to be on an Overspawn Drop mission.

See Also

• Alien Ships
• Cityscape Fighting
• Alien Ship Combat