Actually, Erie serves Savathun, it's easy, so basically Savathun wanted Oryx dead so she tricked Crota into ripping open the dimension which the vex was invading, forcing Oryx to bring his Throne World into his Dreadnaught, but her plan failed, so she decided to convince Oryx to send Crota to the moon and create his netherworld there, and use Eris to lure us to killing Crota which angered Oryx and later he came, then we took him out, now Eris has satisfied Savathun which will make Eris evil and make Savathun strong enough to kill us all.