In the novel Towards Zero, Camilla, Lady Tressilian was a widow in her early seventies. She was the host of her seaside home near Saltcreek. She was found bludgeoned to death in her bed.
In the Christie canon, there are actually two Lady Tressilians, both residing in Devon, and both in works written around the same time. But it is unlikely that they are the same person, or even married to someone with the same title.