The Crystal she takes is what we use to forge one of the exotic sword heavies. It appears Mara Sov sacrificed herself and the awoken to both warn the Vanguard and get Eris to that crystal. What I mean by this is she attacked to get his attention then used her most powerful weapon (the spheres she launched) as a show of force to get Oryx view her as a legitimate threat and have him do the same. In doing so, it provided an early alert of Oryx's presence to the Vanguard led to them taking appropriate action. If she hadn't, it's likely Oryx would have made it far closer to earth and been able to invade in full force and crush the last city. The importance of the Crystal was likely gleaned by Eris through interaction with Toland. Based on its presence as a core in Oryx's sword (and will be a core in our sword) I imagine it has some special properties discovered and utilized by a civilization long lost to the darkness. Toland discovered this in his research of the Hive, sharing this with Eris. Knowing that you killed Crota, Eris likely saw this as a unique opportunity to claim it for the Vanguard. You are a proven guardian. If anyone has a chance of challenging and killing a god it would be you. This is supported by her desperate action to save you during the last rite mission, although that might have just even her doing her duty as a former guardian.