There's a quest to talk to Thonar. After she greets you and you persuade her to let you talk to him, the first dialogue you pick with him (any of the 3) sets off the 2 undercover NPC's and Rhiada gets killed by them unless you stop them in time. Be careful not to hurt her or Thonar in the fight against Nana Ildene and Donnel that turn into sorcerers though, or the guards will come.
Rhiada is married to Eltrys and if you do the Forsworn Conspiracy quest and talk to Eltrys you will find out she is pregnant, this being said, there is no script of this and also doesn't appear to be pregnant or ever have the baby in the game. She is a patron of the Silver-Bloods. If she lives through the attack, Thonar will flirt with her asking about her relationship. This may be a plot to assassinate her spouse in order to have her as his new bride.
- The Elder Scrolls V: Skyrim (First appearance)