In the devils layer after taking down the first mesh layer dinkle bot says something along the lines of;
The fallen are more organised it's like they're attached to ...
I think one of the war minds poss rasputin is coordinating some or all of the fallen
Maybe the ai found a way to survive and that was by joining the winning side and embracing the darkness
English
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Actually right after he says that, an alarm goes off. The fallen are organized by following the alarms your fireteam have set off.