Some very interesting material here. One big thing to note is that there are far more than 5 of those shadowy beings, meaning that they are not, or at least aren't exclusively, Eris' fireteam. While discussing this with friends one of them brought up the possibility that these are all guardians from the Great Disaster, where thousands died. Another theory we have to consider now is that these things may not be guardians at all, but we'll have to wait on that.