Katararenu Noroi no Honshitsu
Unspoken Essence of the Curse

The curse of the Sohma clan. The curse is separate from the transformation into animal form. The curse that Hatori describes as uncanny and insidious...

A more powerful explanation that comes to mind, for the Sohma members, the inidvidual that casts a dark negativity would be the clan leader, Akito. He reigns over the clan with absolute power, and those who oppose him are violently punished. He especially despises when the possessed members come to love others. His unusual and inexplicable actions - are they the source of the true nature of the curse?