The lore in the game says that her Ghost was killed/destroyed during the raid on Crota. Normally when a Guardian's ghost is destroyed they the guardian 'dies' or goes back to being dead. However Eris seems to have survived because she found something in during the raid. I believe the green shard she's holding acts as her new ghost. However, the question is what exactly is the shard she's holding and why does it have the same properties as a ghost. Now we leave the realm of what we know from the game and delve into conjecture. My theory is that the Darkness or Oryx or whatever works in the same way as the Traveler. Where it no longer has a degree of agency and has to summon or create its own Guardians. These guardians would be your various Hive enemies, Knights, Wizards, Thralls, Acolytes etc. Now some of you may draw the conclusion that this means Eris is evil as she is an agent of the Darkness/Oryx whatever, but there have been cases of rogue guardians, that is to say Guardians who don't serve the traveler like Shin Malphur. Alternatively someone thought someone glowing green would be a cool quest giver for the DLC and I'm reading too much into things. Draw your own conclusions and above all else Git Gud