A Widow's Revenge is a side quest in Tyranny, unlocked after Raetommon's attack on the Crossing.
Deya asked you to avenge her wife's murder. Sandro, a member of the Bronze Brotherhood, killed Phaedra and stole her necklace. Deya wants you to find Sandro and get the necklace back.
Travel to Twin Rivers. Sandro is hiding on a small plateau in the north-eastern corner, accessible by climbing (52 Athletics). Talk to him.
If you don't attack immediately, you will be able to pry his version out of him. Subterfuge 52 and Athletics 57 work well. It turns out that Sandro is a witness who saw Deya kill Phaedra. You can investigate the matter and use Athletics 31 or Lore 31 to get a more complete statement (lying by omission is a violation of Kyros' law).
Sandro points out a couple of facts. First, as the granddaughter of Lethian, Phaedra owned the land Lethian's Crossing was built on and thus was the formal leader. She never really paid it any attention, only doing what was needed to get by. Second, Deya desired the land, so she married Phaedra, fooling her that she was in love. She desired the land it was built on and in case of an unfortunate incident, she would inherit the Crossing.
If you don't kill Sandro, you can confront Deya. Asking Sandro to give you the necklace (which wasn't Phaedra's, but bought in Ardent ten years earlier as a gift to a friend) will make prying the truth out of Deya easier. You can give her the necklace and question her on the reasons for her murder. As it turns out, she never liked her, she hated her ever since their first meeting. Her sunny optimism grated on her and the fact that she owned the Crossing only solidified her resolve to eliminate her and seize control.
You can choose to extort Deya for a merchant price cut in exchange for your silence. Or kill her, as is the penalty for disturbing Kyros' Peace.→