During her internal conflict with the dark side of the Force in the Knights of Ren weapon differentiating itself from more familiar Sith lightsabers by having both a crossguard and an unstable red blade. The envisioned double-bladed lightsaber of Rey’s dark side doppelganger has a pair of unstable red blades, and the origin of Kylo Ren’s weapon provides a possible explanation for how such a weapon might exist in Star Wars canon.
Kylo Ren is an astoundingly powerful dark side in the Star Wars sequel trilogy, though he’s a Knight of Ren, not a Sith Lord. His lightsaber, as revealed in Star Wars: The Rise of Kylo Ren, is the same weapon he used as a Jedi, but its Kyber crystal was removed and “bled” via dark side-induced torture, turning it red but cracking it in the process. The crystal became volatile and dangerous, requiring Kylo Ren to build crossguards into the lightsaber, venting the excess energy to stabilize the blade while also providing additional offensive and defensive capabilities.
The hypothetical dark side version of Rey uses a hinged double-bladed lightsaber with two unstable red blades, similar to Kylo Ren’s weapon. The simplest explanation for such a weapon is that it was simply a vision and the lightsaber’s unstable blades don’t need to make sense, but there is a way that the weapon could exist without contradicting the internal logic of the Star Wars canon timeline. Kylo Ren’s lightsaber has a single cracked Kyber crystal, requiring a crossguard to vent its energy, while Dark Rey’s lightsaber could also use a single cracked crystal, with the weapon’s second blade serving the same function as Kylo Ren’s crossguard.
Kylo Ren is most likely a practitioner of Form V, the lightsaber style that his mentor, Luke Skywalker, used as well as that of his idol, Darth Vader. The heavy-handed yet quick combat form is inspired by Kendo, broadsword fighting, and fencing, making the addition of the crossguard helpful for Kylo Ren upon becoming a Knight of Ren. While Rey is only shown using a conventional single-bladed Jedi lightsaber, she’d used a quarterstaff for self-defense for years, making a double-bladed lightsaber a weapon she might have more ease at mastering while its construction would make clever use of a cracked Kyber crystal.
While venting the excess energy of a cracked Kyber crystal into a second blade makes sense within the Star Wars canon universe, it’s also worth considering that, outside of Force visions, Rey has only ever seen Kylo Ren’s unstable red lightsaber blade. This likely makes it the default image of a dark side weapon in her mind, so when imagining herself as the Sith Empress (as opposed to a Knight of Ren), she’d see herself using a pair of unstable red blades, even if the weapon doesn’t necessarily make sense. Had Rey fallen to the dark side in Star Wars: The Rise of Skywalker, her double-bladed lightsaber could have had unstable red blades like Kylo Ren’s if it used a cracked Kyber crystal whose excess energy was vented through the second blade.
