There is a side quest available which sends the Dragonborn to talk to Thonar Silver-Blood. Rhiada will greet them, and one can persuade her to let them talk to Thonar. The first dialogue that they pick with him (any of the 3) sets off the 2 undercover NPCs, who will kill Rhiada unless the Dragonborn stops them in time. However, take care to not hurt her or Thonar when in combat against Nana Ildene and Donnel, or guards will appear and the Dragonborn may receive a bounty.