Aliens

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Aliens are on the Solar System. That's the plot of Terra Invicta.

SPOILERS INSIDE!
Aliens start at the edge of the Solar System. They are disgusting and evil.

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Alien Hate

Once you've killed your first alien and completed an autopsy the 'Alien Threat meter' will be visible in the 'Alien' section of the intel panel. Alien threat is scored on a level of 0 and up, where every tenth level increases the threat level by 1, with 10 - 20 - 30 - 40 - 50 each adding one "level" to the intel panel. It is also measured the same way other factions measure relations with you.

Factions (and aliens) get 1 hate per tier of each module you've destroyed.

Aliens (and their proxy) get 40 hate if you destroy their landed UFO (whether by bombardment or by assault)

Aliens (and their proxy) get 10 hate if you destroy their facility (whether by bombardment or by assault)

Aliens get 80 hate if you destroy the alien nation.

Also:

Aliens gain an additional 1/8th of the hate the proxies get (or 1/4th if they can contact them)

Proxies gain an additional 1/4th of the hate the aliens get (or 1/2th if they can contact them)

Appeasers gain an additional 1/3th of the hate the aliens get if they can contact them

These are all temporary values, and what I'm guessing most people care about is Mission Control. So:

The first thing I noticed is that the Mission Control hate applies to ALL factions. The only caveat is that factions whose x axis is below -1 and factions whose x axis is above 1 have an additional minimum hate of 1.

Meaning Resistance and Humanity First will always have 20 hate for Servants, Protectorate, and Aliens, and vice versa.

The actual formula depends on the difficulty. Cinematic is 0.05 per MC, Normal is 0.4 per MC, Veteran is 0.6 per MC, and brutal is 1 per MC. It's always 0 if your faction is Servants or Protectorate.

It's reduced by 20% (multiplied by 0.8) by Strategic Deception, which is guaranteed to all factions but the Protectorate and the Servants, and another 0.8 by Operational Misdirection (guaranteed for Resistance) and Maskirovka (guaranteed for Initiative), though these are not guaranteed projects for anyone else (50%).

Your councillors will start warning you once you surpass ~42 (50/1.2) hate from aliens (and others) by MC, by the way, which is equivalent to the base hate from 105 MC. The cap for 50 hate is 120.