The nature of dark side Rey

Question

When Rey finds the wayfinder a dark side version of herself comes out. Is it just her hallucination, or something triggered by her presence?

Answer 1

Rey herself recognises that the apparition is a Force Vision (e.g. not physically real). It was triggered by her touching an object that had been possessed by a strong Force user, in the same way that touching Luke's lightsaber in Maz's Palace made her have visions of the Jedi .

A hooded figure materialized, glided toward her with inexorable purpose, dark cloak sweeping the ground. The figure practically radiated power, and something else…a cold and ravenous hunger.

A red lightsaber appeared in the figure’s hand, chaotic like Kylo’s, with two parallel blades. Light from the blade finally illuminated a face as pale and gaunt as it was fierce.

Rey gasped, stumbling backward. It was her. Her face, her form. Cold and dark, wearing a Sith cloak, whole at last.

Horrified, she watched as the dark mirror Rey swung her blades apart, forming a long, fiery quarterstaff. It was the very saber she’d tentatively begun designing in her mind.

This couldn’t be real. It was a vision, nothing more. But the dark Rey’s steps echoed when they met the floor, and her lightstaff reeked of ozone. Her power was incredible, intoxicating. Almost against her will, Rey began to reach with her hand…

_{^{The Rise of Skywalker: Official Novelisation}}

Answer 2

It was a Force Vision, much like when Luke's experience in the Dark Side Cave on Dagobah. As I understand it, there is not a physical reality to it, but a warning of what she could become if she fell to the Dark Side.