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 Air Raid 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 | Air Raid | 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
| 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.
Base Assault Fleets
| Mission | Primary | Escort | Week |
|---|---|---|---|
| 1 | | 5 | |
| 2 | | 5 | |
| 3 | | 5 | |
| 4 | | 5 | |
| 5 | | 5 | |
| 6 | | 5 | |
| 7 | | 5 | |
| 8 | | 5 | |
| 9 | | | 7 |
| 10 | | | 8 |
- Subversion fleets can appear from Week 5 onwards.
Air Raid Fleets
| Mission | Primary | Escort | Week |
|---|---|---|---|
| 1 | | 4 | |
| 2 | | | 4 |
| 3 | | 5 | |
| 4 | | | 4 |
| 5 | | | 5 |
| 6 | | 6 | |
| 7 | | | 7 |
| 8 | | 9 | |
| 9 | | 9 | |
| 10 | | 8 |
- Attack fleets appear from Week 4 onwards.
Overspawn Drop Fleets
| 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.