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Its going to be long so under the cut it goes, not really grammar checked and sorry if my ramblings dont make sense
Okay so my selkie/mer aus, i have two aus, but both have selkies
And I say selkie/mer au because i love selkies so much, and i also love fun mer aus, so i have two
But for the aus they do have one thing in common which i like, the kings being called sea kings(cause you know,,,, the seven seas) and the nobles and king being called pod
For each au they have different species but some will remain the same cause they just match so well(Like Sitri being a leopard seal selkie)
Selkies can be seals or sea lions, because they are both similar but different and also super fun
But also hybrid are fun, and for selkie au the selkie genes are dominant, and the children born from a selkie will be born a selkie, but if its a mix of a mer and selkie, they will still have some traits from the other parent, like shark mer and selkie would make a selkie that has even sharper teeth that can grow back and sharper claws and double peen if you want(also egging cause hehe)
And im stealing some fun things ive seen with selkies, like having three forms, an inbetween form of seal and human, but fur still all over and a human face, selkie spots that can match their pelt
For Belphie i gave him the Ribbon seal(which is a cool as fuck seal) and his tattoos are selkie spots that reflect the pattern on his fur, and also somewhere in his line someone mated with a remora mer and so yes Bephie can egg people up, and he is an extra lazy seal
For Satan as a harbour seal the barcode tattoo he has is replaced by spots of the pattern of his fur, and the cross on his forehead is there but looks more like a spot, like not just two lines but like, fuzzy like it was painted on by a paint brush with those fuzzy edges, but the cross is still there in seal and inbetween form
Mammon is an elephant seal selkie in the selkie au, the largest one and his markings are both on his seal form and human form
For Leviathan in the selkie au i gave him the mediteranian monk seal because they are rarer and also pretty cute, so still larger than Satan’s harbour seal
I dont have the other kings ready for selkie au but i do for mer au
Mer au i may combine some different types of species for the kings, but they will be within the same genus type
Like in my post before Beelzebub is a mix of a gulper eel and a moray eel. An eel without a ribcage and an eel with two mouths, such fun and gluttony
Satan is a mix of a great white and a thresher shark, a shark with a long tail that stuns their prey and a great white because he do be pale as hell
Mammon i’m debating on a koi fish or a gold fish, but i’m having him as a koi for now, and yes he is still bigger than Satan, nice pretty shimmering scales
Leviathan is an pacific giant octopus of course, so fun tentacle fun there
Asmodeus I like as a seahorse for the fact we will get HIM pregnant, but still deciding on other fish for him(seahorse is not confirmed)
Belphie in mer au i might have as still a selkie, cause its fun with the ribbon seal stuff
Lucifer is an orca, cause dolphins and orcas are bullies of the ocean, so fallen angels and angels will be in the dolphin and orca family, plus they use mobbing to bully other animals and can be found everywhere so its perfect that they can be found everywhere to attack the other sea kings
I love dolphins and orcas but they are the apex predators of the ocean, and they use their intelligence to bully other animals
For selkie au i might add in selkie colonies because the former Nifilim selkie colony where only Agares and Vassago survie could be fun, because orcas eat and kill seals and sometimes for no reason
Also for both aus, i may combine something from the whb game where they have a second bigger form called sea king form, still debating rn, but it’d be like how Satan before changed to his other form or Belphie to the giant skeleton form, but essentially means they can have another form and it can be a different type of species too maybe
Also they have some water magic like how in Mako Mermaids they cna control water, I do like that part, some are more gifted in the magical sense, some more just of the physical kind
But this is all i have really in my head rn for the aus, and reader and mc can be many different things, another mer, a human descended from a human that helped them way long ago(cause of course mer and selkie lifespans are different), the possibilities are endless 
Also can have some fun cxc too maybe, but i love mer and selkie aus so much, they rule my heart