Due to the rumors that Shadowkeep will have the Black Garden as its raid I hypothesize two things: 1) It is a Vex expansion masquerading as a Hive expansion. 2) The Nightmares are the Vex answer to the Hive's power to Take. Eris Morn accidentally freed some sort of fragment of the Darkness. The Vex stole it for themselves. And then used it to show the Hive that they too can wield the Darkness on their own. I recognize this is unlikely but if it were true it would be a hell of a plot twist. Originally the Vex were ominous. An evil that detests other evils. From the Taken King onward that changed. Now their mastery of the Darkness is being portrayed as amateurish relative to the Hive. The plot twist I describe would be the Vex's "am I a joke to you?" moment. The moment in which everyone in the Solar System has to reconsider how much of a threat they are. After all, Guardians can't be Taken. But are they immune to the power of the Nightmares?