Castifer has decided to visit Heaven to terrify a lot of nervous angels and play lots of head games with them to try and get them on side
I actually kind of like this… it could work well with a lot of world building of angels who are, ultimately, a species that has (canonically) no free will and is so used to serving a higher power. Now all the Archangels are dead…. Except one. One they’re also terrified of. And just after they’ve thrown their greatest power at Amarra and she hasn’t been all that bothered by it.
Not all that – but she is still injured by this big nuclear heavenly wrath. But Rowena is there to help heal her
Yes, Rowena is still alive, because she is Rowena, and she always has a plan B, plan C and possibly a plan PQRST1. She’s also on Team Amarra wanting to be by her side while she takes on all the powers of the world and remakes the world in her image.
That is until Amarra, healed, decides to send a warning shot against Heaven, terrifying everyone (including Rowena) with a tiny flick of her awesome power. Rowena perfectly acts that horrified moment when she realises how utterly over matched she is and quickly tries to switch sides.
That other side involves Crowley – who has slaughtered some random disposable Saudis to get another Hand of God weapon to share with the Winchesters, but we have some arguments over how to use it:
Sam wants to focus on Amarra, and if that means Castiel being Castifer for a bit longer, so be it
Dean wants to take down Amarra – but only after freeing Castiel
Crowley wants rid of Lucifer because he has an EPIC GRUDGE then they kill Amarra
Sam rightly points out everyone’s priorities are somewhat skewed but since Crowley is the one with the Hand of God he chooses. Their plan to put Lucifer back in his cage requires Rowena and the book she hid – thankfully she’s willing to come out of hiding and join side
And it all goes wrong – because Castiel is in such thrall to Lucifer (who is, again, the most powerful of the Archangels, possibly on par with Michael) that he can’t come close to getting rid of Lucifer (though is this a moment where we mention, again, that this isn’t ACTUALLY Castiel’s body). Even when Crowley tries to help with a threeway possession. Everything fully screwed, Crowley runs away, leaving Castifer free to take the horn and prepare to smite the Winchesters
Which is when Amarra arrives.
Time for a dramatic show down as Lucifer gathers all of his vast power and all of the power of the Horn of Joshua to smite Amarra with all his utterly awesome power… and does nothing
Doesn’t even make her hesitate. Sam and Dean put this down to the Hand of God needing to be wielded by “god’s chosen” and ye gods Lucifer must be at the VERY BOTTOM of that list. Personally I want to raise the possibility that if the combined wrath of heaven (which is presumably at least a bit god touched) only makes Amarra hesitate, then maybe stuff he touched once don’t really carry the power to bring down his sister. Especially since she thinks she can win a one-on-one fight against god himself… maybe pinning all their hopes on these items isn’t the way to go
Amarra doesn’t kill Castifer though – she figures if anything will get her god’s attention it will be torturing his first and greatest son…