A common theme is the power struggle. Castiel is often seen as a more 'pure' angel, while Lucifer is the rebellious one. Their interactions are filled with this underlying battle for dominance or influence. Another theme is redemption. Sometimes Lucifer is portrayed as having a chance at redemption, with Castiel being a key factor in that.
A common theme in such fanfiction is the struggle between Castiel's sense of duty and Lucifer's chaos. Castiel is often seen as a dutiful angel trying to do what's right, while Lucifer represents the breakdown of order. Fanfictions may show Castiel being conflicted when dealing with Lucifer, as he may see some of his own doubts about Heaven's ways reflected in Lucifer's actions.
In some fanfictions, the relationship between Castiel and Lucifer is depicted as a complex rivalry. They both have strong connections to the Winchesters, but for different reasons. Castiel wants to protect them, while Lucifer has his own dark plans. This creates a lot of tension and interesting plotlines where the two angels' paths cross and they engage in battles of wits and power.
One popular 'castiel lucifer fanfic' is 'The Fallen and the Faithful'. It delves into their complex relationship and how they interact in an alternate universe. Another great one could be 'In the Shadows of Heaven', which explores their power dynamics and the moral gray areas between them.
Some fans would be extremely shocked and heartbroken. Dean and Castiel have a very complex and beloved relationship in the fandom, so seeing Castiel kill Dean would go against what many fans hope for in their relationship. They might be very vocal about their disappointment.
One possible plot could be that Castiel is under some sort of dark spell or mind control that makes him turn on Dean. Maybe a powerful demon has manipulated him, leading to a tragic confrontation between the two friends.
There were a few Xianxia novels with decisive protagonists that were worth recommending. Amongst them," The Battle of the Great Dao " was a fantasy Xianxia novel that focused on the fight for resources. The protagonist was decisive in killing, both wise and brave, and constantly worked hard to pursue the Great Dao. The plot of this novel was compact, the writing was smooth, and the characters were lively. It was suitable for readers who liked traditional Xianxia novels. In addition," Immortal Mortal " was also a Xianxia novel that was decisive in killing. The protagonist relied on his methods and brains to advance bravely, cutting through the clouds and thorns, showing the image of a strong person. The plot of this novel was reasonable and the writing was excellent. In addition," Evergreen Immortal Dao " was also a Xianxia novel with a decisive protagonist. The protagonist took one step at a time in the world of cultivation, breaking through all kinds of difficulties and finally achieving Dao. This novel was based on the cultivation of the family. The plot was plain but charming. These are the recommendations of a few Xianxia novels where the main characters kill.
Power is a major theme. Two Lucifers would bring a lot of power into one story, and it can be about who has more power or how they use their power together.
Castiel's relationship with his powers might also change. He may have to limit his use of angelic powers to protect the unborn entity. This can cause internal conflict as he is used to using his powers freely. At the same time, it can also show his growth as a character, as he is willing to sacrifice some of his power for the sake of something new and precious.