Rin is identified as the new Captain of the Guard of House Davar whilst Shallan is still a child. He is instructed by Lin Davar to kill Helaran - Shallan's eldest brother and Lin's eldest son - where Helaran is supposedly residing in Valath, a city in Jah Keved. Shallan's father indicates that Rin should bring him Helaran's head, that he should kill Helaran before he summons his Shardblade, and that said Shardblade would be Rin's payment so long as he served House Davar.
Complete:This page has sparse information from the books. However, it does contain everything we have on the subject at this time.