The unique ring, Sway of the Master (Kapaku), only seems to be equippable on Kapaku Heroes despite not having the usual "Only Equippable on Kapaku Heroes" text in the description. Other faction-specific items bear this text such as Spheroid of Drakken Lore, Deep Diviner, Quivering Circlet, Order of Isivier, Wordless Will, Incubation Orb, etc.

On the other hand, I'm not sure whether the faction exclusivity is or isn't intentional.