(covers information from several alternate timelines)
Ceti Alpha VI was the uninhabited sixth planet of the Ceti Alpha system. This system was located in Sector 25712 of the Alpha Quadrant. It was located a considerable distance from the Delphic Expanse and the planet Denobula as well as three days away from Regula I at an unspecified warp speed. The nearest starbase was Starbase 12. (ENT: "Twilight"; TOS: "Space Seed"; Star Trek II: The Wrath of Khan)
The planet exploded for reasons unknown in 2267, approximately six months after Khan Noonien Singh and his followers were exiled on Ceti Alpha V. The shock of the explosion shifted the orbit of Ceti Alpha V, destroying that planet's ecosystem, making the planet virtually uninhabitable.
In 2285, the USS Reliant arrived in the Ceti Alpha system to find a suitable planet for testing of the Genesis Device. Pavel Chekov and Captain Terrell beamed down to Ceti Alpha V, mistakenly thinking it was Ceti Alpha VI. Just before they were captured by Khan and his Augments, Chekov realized that they had made a grave error, discovering that the cargo containers they'd found belonged to Khan's ship, the Botany Bay. (Star Trek II: The Wrath of Khan)
In an alternate timeline, Human refugees fled to the Ceti Alpha system, following the Xindi's successful destruction of Earth in February 2154, during the relentless pursuit of the Xindi to eradicate all Human colonies. (ENT: "Twilight")
This planet was only mentioned in dialogue.
This planet's quadrant of origin is inferred based on the position of Starbase 12, the closest starbase, as seen in the star chart appearing in the Star Trek: Discovery episode "Magic to Make the Sanest Man Go Mad".
In Greg Cox's novel To Reign in Hell: The Exile of Khan Noonien Singh, Spock theorizes that a small black hole might have passed through the system, too small to be detected easily by the sensors aboard the Enterprise, and that its immense gravity destroyed Ceti Alpha VI and fundamentally altered the orbit of Ceti Alpha V.