Sou'unga
…
Worse
Kagome had thought Tokijin was bad; Sou'unga is far, far worse.
Fear
She had always been subliminally aware that there was something nasty lurking somewhere in the shrine she called home. Her childhood nightmares had occasionally featured a putrid, carmine-eyed monster made entirely of endless strips of rotting flesh. That the creature that had terrorized her nights actually existed and inhabited a sword Inuysha had found in her home nearly broke her entirely. Homes were supposed to be safe, not have evil swords lurking in them to ambush your friends.
Defiance
Tessaiga's fierce defence of his wielder -and the hanyou's own pig-headed stubbornness- was all that prevented the evil blade from taking Inuyasha over completely. As it was the dog-eared boy lost control of his arm to the thing and went half-berserk. Only Miroku and Sango's quick thinking prevented a massacre.
It was however Sesshoumaru who -with a little help from Kagome- managed to separate Inuyasha from the terrible blade.
Just being near the disgusting thing gives the miko the creeps: Nothing should look that foul and still be walking around.
Cooperation
In order to defeat Sou'unga Kagome must first achieve a far more difficult feat: get Sesshoumaru and Inuyasha to stop bickering and work together for once. She fails miserably, but they do finally stop fighting when she and Rin are abducted by a former foe of the Dog General's that the evil blade has resurrected. They even manage to rescue the two girls together.
Kagome isn't really surprised Sou'unga used to be Inutaisho's sword; Sesshoumaru had to get his taste for evil, body-snatching swords from somewhere and his father was the most likely candidate. Tokijin is just as evil as Sou'unga, but far less powerful.
Return
After Sou'unga is defeated the Dog General's spirit speaks briefly to both his sons before vanishing again.
At least, appearing to vanish again. Kagome can still see him just fine; he looks like a cross between Tenseiga and the original Tessaiga, proving he is the prototype both blades hold echoes of.
That he has somehow attached himself to the time-travelling miko is again unsurprising; she has had his youki running through her aura and filling the gaps in her soul for months now. What is upsetting is that Inutaisho has taken back most of his essence from both blades, using their energy to anchor himself to Kagome.
Both sword-spirits re-emerge after a day or so, but they are now as transparent and ghostly as Tokijin, a far cry from their former selves. Tessaiga will never again be able to touch her.
Replacement
Kagome does not get a chance to miss Tessaiga: Inutaisho smoothly picks up where the sword-spirit left off and is both more solid and more thorough in what he does with her. Tessaiga only taught her things relevant to her survival; Inutaisho even tries to help her with her homework. The sword-spirit no longer approaches her, instead drifting around Inuyasha like Tenseiga does around Sesshoumaru and seems to have forgotten her almost entirely.
Inutaisho explains -another difference: he can talk in her head- that he has taken the fang's memories of her as the blade was never intended to act as it did in the first place. It was a sword, not a bodyguard; the Dog General's reluctant death had left pieces of his soul attached to both his fangs and he had taken those pieces back.
Protecting Kagome is supposed to be Inuyasha's job, but the dead daiyoukai recognises that his son lacks the temperament, training and experience to do a proper job of it and offers to take Tessaiga's place in caring for her. Recognising finally that it had been Inutaisho caring for her all along -it was his soul after all- Kagome accepts.
Demand
Nothing good ever comes of the words 'we need to talk'. That it is Sesshoumaru who says it to her fills Kagome with a sense of dread. Her new guardian just smiles, rubs her shoulders reassuringly then gently pushes her forwards after his eldest son.
Finally, Sou'unga and a major turning point. Inutaisho returns!