Siren's Lament

When the snatchers capture the trio, Voldemort is determined to keep his Horcrux from running away (again). A ritual solves the problem; Harry can't very well hunt Horcruxes from a fish tank. At Hogwarts, Neville navigates the Dark Lord's new stratified society as Gryffindor's only pureblooded Heir. Smarter!Voldemort. Prisoner!Harry. Creature/Merman/Siren!Harry. Political!Neville.