From what i understand, the fallen antennae are broadcasting a call to arms, looking to reunite whatever stragglers from the old houses that have yet to join up. The Hive are not-so-obviously calling/reporting to Savathun, Oryx's sister. Many of the Hive scannables mention her by name, as well as all the enemies with names/titles referencing her. The Taken might be doing that as well, based on the enemies named after Quria (a Vex Mind that Oryx Took, and later gifted to Savathun) but I'm not so sire about that yet, because only Oryx had the power to control those corrupted by the Deep. As for the sea monster, i have no clue. I'm kind of hoping it's the Leviathan that resided in the oceans of Fundament, the Hive homeworld. But it could be one of the Worm Gods, seeing as how the Hive have taken a special interest in that planet. That one doesn't make as mich sense tho, because the Worm Gods, as far as i understand, are still trapped on Fundament, living off of the tributes of the Hive.