Memory Lost to the Shadow

Runin, an Elfling who was captured and tortured by Orcs has no memory of who she is or her home. She was living in Tharbad with the Dwarves of Erebor when they return from the quest to reclaim their homeland, they venture back to Erebor with her and the other Dwarves, but why does Lord Elrond stop them and force the group to go to Mirkwood? Also why is he so fixated on Runin?