The Cabal don’t really have that close of a relation to the Darkness. They mostly just follow it to track down the Traveler, from what I’ve gathered. The only species that gain power from the Darkness are the Hive, Vex, and the Taken.
Eliksni (Fallen) follow the Darkness for much the same reason.
Yeah, but they reset after a week. They are hugeeeee ass missions, basically big cinematic experiences that have super powerful enemies with super powerful rewards.
For some clarification on that, Destiny 1’s first raid had you fight a massive Vex named Atheon, and during the fight he would transport you and your team to different points of time. He could literally write your teammates out of existence.
Another raid in Destiny 1 had you fight a sword wielding God named Crota, who had the power to steal your soul with his light-devouring blade.