Let's think about it. Most of them are shadow themed in some way.
Danger Missions that have shadow theme: Legion of Almire, Heart of Ice, Ghost in the Machine
Events: Shroud of the Apocrea, Black Kats, Fiendish Tortodrone Invasion, Dark Harvest, Winterfest*
Boss: Firestorm Citadel
Then there's everything else:
Danger Mission: Compund 42
Events: Winterfest*, Caketastrophe
Boss: Crimson Hammer
What gives? Where's the T3 monster homing wolvers from yore?
Oh yeah they got turned into shadow fiend Gorgos...
Because Shadow is Eeeeeevil!
Also, the Swarm is normal damage. So there's that.