Leviathan is the name given by some Inuit Cultists to a child of Cthulhu: "...the Other Tornasuks left or went to sleep with Him [Cthulhu], except Dagon his servant-creation and Leviathan his son." It is not clear if it is one of the named sons. It seem likely that Leviathan is identical to Behemoth and hence B'Moth.
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Leviathan in 2018 game Call of Cthulhu