(covers information from an alternate timeline)
She was a descendant of the crew of the USS Defiant which had crashed on the planet over two hundred years previously. She ceased to exist when the Defiant avoided the crash and therefore canceled out that timeline.
Parell chose to live as a Klingon among Worf's descendants and others who chose to live this way as part of the group Sons of Mogh. Prior to her non-existence, she, along with Brota, asked Worf to kill them rather than allow them to merely cease to be. (DS9: "Children of Time")