NOTE: Requires a new game for new dark events to take effect. 

ADVENT's gotten tired of XCOM spontaneously developing new and even more dangerous abilities and have been working on new plans to counteract XCOM's advances. 

These include:
Avenger Watch: Every mission has a 50/50 chance of spawning a UFO to try hunting down the Avenger. 

Alien Worm: Intel is halved, and medical, psionic, and proving grounds equipment are much less efficient at doing their jobs. 

Advanced UFO Hunter: Like the Hunt XCOM event except it's guaranteed to hit if it activates. 

EOD Armors - Provides the Blast Padding perk to some ADVENT soldiers, providing them with a point of armour and 66% damage reduction against explosive damage. Will probably cause very inconvenient situations if you have the Grenade Damage Falloff mod installed. 

Lightning Drugs - ADVENT tries out experimental drug therapies on its own soldiers. On the few that it'll work, they'll gain lightning fast reflexes. As in they get the Lightning Reflexes perk. Will probably cause lots of anguish when RNGesus decides to give Lightning Reflexes to a stun lancer. 

Holo-targeting Rounds - Modified smart rounds that function like the holo-targeting perk. Will definitely cause anguish if you let an officer do their marking thing in addition to this. 

Special Forces Surge - ADVENT integrates spec op units into its main field operations, making XCOM face some units who have similar abilities to them. On its own the added abilties aren't too bad, but if you end up letting it combo with some dark events, this could cause some serious damage to your operations. 

Hazmat Armour and Mindshielding Dark Events! ADVENT now has the chance of becoming immune to your war crime grenades and ammo, and they along with the aliens can now roll out their variant of Mindshields against your psi-ops and flashbangs. 

Permanent Dark Events! These only have a chance of being in play when you're in the max doom state, but they have a very high chance of being added compared to regular dark events when they can be added. 

These currently include the terror units (Faceless, Berserkers, and Chryssalids), along with a Permanent Avatars on missions Dark Event. 

Variations of the Viper Rounds event, including all the other rounds in the game. Now ADVENT can potentially set your troops on fire, crit far harder, pierce your EXO/WAR suits' armour, and just have better aim overall. 

Overwhelming Force, adding in multi-sectopod pods on missions when active. 

Berserker Packs! Like the Faceless and Chryssalids, these can now be deployed on non-retailation missions once you encounter these units for the first time. Come in packs of 4 when deployed. 

Riot Control! Realizing how much XCOM seems to detest these units, ADVENT may decide to start forming additional squads comprised of almost nothing *but* stun lancers and deploy them in operations they believe XCOM may try interfering in. And then you realize they've decided to add a shieldbearer among them. 

Early Avatar Deployment! Been delaying the strategy side of things for a while? Maybe let things go all the way towards...I dunno, September or so? 
While your efforts will always prevent mass production of what the Avatar Project is aiming for, low rate initial production is still a thing, and eventually they will start considering to push that side of things to combat deployments if XCOM's lasted long enough without going down. You probably shouldn't let this one go through if it ends up popping up. 

Not included: 
High Alert. This event is from True Concealment, you need to check that mod's page to see how to turn it off. 
Extends X2StrategyElement_DefaultDarkEvents and X2HackReward but does not override. Also makes major modifications to XComMissions.ini to the MissionSchedules side of thing, is very likely to not work with mods that also alter that part of the .ini. 

Has an adjustable .ini called XComDarkEvents.ini. Located at (wherever XCOM 2 is installed)\steamapps\workshop\content\268500\626109271\Config 

Thanks to S. Flo for making the custom dark event images! 
Thanks to thunderbeast for making the Korean localization!
Thanks to Klavi for making the Russian localization!