Castiel's (Misha Collins) backstory is explored in the latest episode of Supernatural Season 12. Lily Sunder comes back to meet with Cas.
According to TV Line, Lily Sunder (Alicia Witt) comes back to seek for Castiel in "Supernatural" Season 12 episode 10. Castiel had explained to Sam (Jared Padalecki)and Dean (Jensen Ackles) that before the apocalypse angels did not come to Earth. A rare event happened in 1901.
This was the time during that era that the angels including Castiel came to Earth because one of their own married a human who happens to be Lily. The angel and Lily had a niphilim, a daughter of an angel and a human. Castiel killed their brother because of this incident. It is a betrayal to the angel's oath in "Supernatural" Season 12 Episode 10.
A report in Entertainment indicates that Sam, Dean and Cas have been searching for Kelly Kline and her unborn child. This is because the father of the child is Lucifer and they need to stop the baby from being powerful once it is born.
Lily comes to Earth to take on her revenge towards Ishim. Her child has been killed so has her angel lover then. She had been devastated with all the events that happened during that era. Cas try to help her by killing Ishim in "Supernatural" Season 12 Episode 10.
Meanwhile, the next episode for "Supernatural" Season 12 Dean. Den of Geek indicates that the next thing that Cas and the team will do is find Kelly's child. There are to be more challenges for the characters as they go on and search for Lucifer's child.
They need to locate her and the child in "Supernatural" Season 12. The only way to do right after the baby is found is to kill it. The niphilim cannot be born since there will only be destruction.
