Cydian was actually pretty young as a turian before being turned, but has been a Nocturni for quite some time. She’s been around for a century or two already, but she hasn’t really been all that bad (in her eyes).
She was just finishing up her service as an engineer when the ship she was stationed on was attacked by pirates. The ship was a small cruiser and was caught by surprise, unable to defend itself before the pirate vessel damaged their shields and forced a boarding.
The pirates were not simple turians, however. They were Nocturni. Hungry Nocturni. They fed on the crew and Cydian was left bleeding to death in a pile of her fellow crewmen when the leader found her. He was a sadistic man, but saw her still trying to crawl to safety and thought she’d make a nice addition to his (hate to say it) harem. He had females he kept near starved, weak, and completely reliant on him. She became one of those females.
She was kept prisoner for quite some time and her captor eventually became complacent with her and the others. During this time, Cydian made two ‘sons,’ her ‘first born’ she calls them, during a similar assault on a colony. Instead of taking then prisoner, she offered them a chance to live and they took it. Becoming loyal to her, they built their strength until the time they banded with her and the other females to turn the tables.
They slaughtered the pirate gang, but it cost quite a few female lives because they were so weak (Cydian’s sons were feeding her whenever they could). What few remained either hated their existence and refused to continue or joined her.
Over the years, the only ones to survive are Cydian and her sons.