How about this?

"So far, they were just honor guard.... "
Like the Faceless and Chryssalids, Additional Archon Patrol now be deployed on non-retailation Most missions.

I've got onee:

Firewalls: Increases the tech score of all robotic enemies for a month.

I will think of some more later.

An idea for a new dark event.
Psionic impovment.
Special buff for all psionic enemies (mind shield and an ability to cast on themselves an resuretion, just like psi-operative). So, they not die, they will resurrect with 3 or 6 hp and move to a new cover. Btw, they will get a buff, for their aim / defence AND one more action!
Super harcore dark event)))

I have some more.

1. Muton Deathsquads: Once active, a squad of three mutons will be called down as reinforcments for each mission, lasting a month.

2. Armour piercing: Advent troops (Troopers, Officers, Stun Lancers and Shielbearers) gain Armour Piercing rounds, on every mission for a month.

*The Creator already made the second one, and i wasn't aware, but its there is guess*

3. Heavy Armour: Mutons will gain an extra two armour for the rest of the month.

4. Encampments: Turrets gain a bonus to their aim, defence and tech score, they also appear twice as much. Lasting the month.

5. The Avatar: An Avatar will appear in your next mission.

6. Retaliation: Will Launch a Retaliation mission insantly, with twice as much troops as normal.

7. Sectoid Training: Sectoids have a random psi ability from the psi ops pool, each Sectoid gets this ability for a month.

8: Missile Strike: Missiles will be called onto your position during battle, will happen once a mission and lasts a month. (Works like the Archons blazing pinions attack)

9. Message to the people: Civilians will have tripled dection radius and will patrol, trying to help the advent. Lasts a month.

10. Psionic Immunity: Grants all psionic troops (Sectoid, Codex, Gatekeeper and Avatar) Immunity to fire, posion, acid, mind control, panic, disorenitation, critical hits and rupture for a month. (Have fun with that Commander)

11. Sectoids: Activates a Gurellia Mission with only Sectoids and Advent troopers. The Sectoids are immune to negative effects and have extra aim, defence, health and armour.

12. Sectopod Targeter: For a month all Sectopods automaticly enter overwatch at the end of their turn.

13. Will of the Warrior: All Enemies gain 100% more will for a month.

14. Beam Rifles: All Advent use beam rifles for a month.

15. Domanation: Sectoids and Avatars gain the Domanation ability for a month.

16. Armour Stash: Every enemy gains three armour for the next mission.

17. Assassin: One of your engineers is killed. (Another version of this for scientists)

18. Ever Vigilant: All enemies gain the Ever Vigilant skill for a month.

19. Masters of Melee: Stun Lancers get Bladestorm, Blademaster and Reaper for a month.

20. Unknown Event: When completed picks a random dark event to become active, meaning you won't know what it is until it happens.

Good Luck, Commander

1. Dimensional Anomalies, all Codex gain 2+ armor, gain 4 HP, heal 2 HP everyturn, clones start at the health that the Codex that it cloned from had during cloning, instead of spawning with half health. Along with that, more Codex will appear in all missions for the next month.
2. Supperior Arms, all Advent forces gain bonus 5 aim, and they all gain access to a grenade for a month.

Here's some ideas:

1. Orbital Scan
Causes an immediate retaliation strike as soon as the Dark Event is completed, regardless of the upcoming one's timer.
"The last thing the resistance needs is yet another punch to the gut." -Bradford

2. Insomnia Virus
Deploys a virus onto the Avenger that slightly decreases all soldiers' Aim and Will stats and all construction/research bonuses from engineers and scientists until a cure is researched.
"Our people need to be fit. How can they fight in such a sorry condition?" -Tygan

3. ADVENT Airstrike
For a month, once per mission, during the aliens' turn (after concealment has been broken), several missiles will home in on your soldiers' current location, which will land the following turn. This is functionally identical to an Archon's Blazing Pinions attack.
"If there's one thing ADVENT has that we don't, it's their far superior air force. Heads down." -Bradford

Originally posted by Lord Sipia:

Here's some ideas:

1. Orbital Scan
Causes an immediate retaliation strike as soon as the Dark Event is completed, regardless of the upcoming one's timer.
"The last thing the resistance needs is yet another punch to the gut." -Bradford

2. Insomnia Virus
Deploys a virus onto the Avenger that slightly decreases all soldiers' Aim and Will stats and all construction/research bonuses from engineers and scientists until a cure is researched.
"Our people need to be fit. How can they fight in such a sorry condition?" -Tygan

3. ADVENT Airstrike
For a month, once per mission, during the aliens' turn (after concealment has been broken), several missiles will home in on your soldiers' current location, which will land the following turn. This is functionally identical to an Archon's Blazing Pinions attack.
"If there's one thing ADVENT has that we don't, it's their far superior air force. Heads down." -Bradford

Number 1 and 3 sound similar to mine...

I wish you could or knew how to actually add NEW things to dark events. I have a great idea for one! Its call, "Big Mother Bugger!"

as a retaliation tactic, the Aliens have found a way to JUICE up a Chryssilid (much like how the Berserker has been juiced) and this new Queeen Chryssilid is 3 times the size and has 5 tims the HP. She could have a few new skills... but when she kills a human and they become a caccoon, her new caccoons produce 6 new standard Chryssilids (2 a turn) instead of the 1 a turn.

only 1 Queen can be spawned in a mission (if spawned) for that month.

this would be the effect IF we dont counter it.

Idea suggestions?
Can't think of a good name, but I like the idea of pods of 3 Gatekeepers spawning, or at least, adding more Gatekeepers to the game.
Maybe something along the line of "Psionic Anomaly" or "Gate Rift"

How about this one,
Orbital Assault incoming advent reinforcements no longer signal with the red smoke, and no longer trigger overwatch.

Dark Events that give some enemies perks for example Run&Gun or Squadsight would be interesting.

Covert Countermeasures -

All missions limit you to 1 less soldier.

Covert Counterintelligence -

All missions start fully revealed unless 2 less soldiers are brought along, thus starting the turn-timer with true-concealment.

Real Men Skip Vanilla -

All default dark-events that came with the game are disabled cause they are too easy.

events to work with custom enemies like the Advent Sniper, Muton Centurion or the like would be neat

for vanilla suggestions

Alien Weapon Mass Production-
Advent Troops are upgraded from Magnetic to Plasma Weaponry

On-Board Repair Drones-
All Mechanical Enemies get Repair Servos

Lethal Lancers-
"Stun" Lancers upgrade to Arc Blades

Heat Lancers-
"Stun" Lancers upgrade to Fusion Blades

Not sure if you can do it or not but wanted to throw it out there,
Air Superiority: numerious UFO's force avenger to ground and must be defended. instant action

Air Superiority sounds like a great dark event!