the self knower.
''The transcendent intelligence attained intuitively by the tathagatas by their self realization of noble wisdom is a realization of their self nature.'' (Lankavatara Sutra).
in order to attain and perceive non duality there must be a complete dissolving of the individual ego first.
the Buddha nature is a form of self, emptiness is a form of self, a knower, however even own nature is impermanent (contradictory to some scriptures). (see teaching about the great icchantika, master keizan's transmission of the lamp. )
emptiness/own nature/universal mind talk is most important, rather than talk about universal mind its more specific to talk about emptiness and the awareness that perceives it. awareness can be seen as the self knower. the pupil within the circle of emptiness.
best wishes, Tom.
in any matters of importance. dont rely on me. i may not know what i am talking about. take what i say as mere speculation. i am not ordained. nor do i have a formal training. i do believe though that if i am wrong on any point. there are those on this site who i hope will quickly point out my mistakes.