Y/n finds himself thrown into a spacial gap and is forced to fight a Demon(Yokai). During the fight he unlocks his Sacred Gear, a Twice Critical(seeing as how its shown that not all twice critical looks the same or act the same, Y/n's Twice Critical manifest as a Pair of Gauntlet Bracers, Grieves, upper body armor, and head gear) the place he is fighting the demon(yokai) is the Bone Eaters well.

Y/n's Twice critical can assimilate energy and power into it, examples are, Sheikon Jewel shards, a Lightning blast from a Raijin, the Blood of a Daiyokai Inugami, the true body of a Tsukunogami(binds the Tsukunogami to Y/n)) effectively giving new abilitys and sub-balance breakers.

Harem
Fem Inuyasha, Kagome, and Sesshomaru(combine the fact shes female and a dog, she'll have more maternal instincts, making her dislike the Unmother plan even more, and imprint onto Rin, adopting her)