The disaster in which Ceti Alpha VI exploded six months after Khan's exile, shifting the orbit of Ceti Alpha V and turning it into a desert.
The devastation of Ceti Alpha V is recounted by Khan in Star Trek II: The Wrath of Khan. In the episode Space Seed, Kirk left Khan Noonien Singh and his followers on the planet, a harsh but fertile world. Six months later the neighbouring planet Ceti Alpha VI exploded.
The shock altered the orbit of Ceti Alpha V and laid everything waste. Khan's people survived in cargo containers amid constant sandstorms. Twenty of them, among them his wife, were killed by the Ceti eel, the only native creature left.
No one from Starfleet came to check on the exiles. In 2285 the USS Reliant, taking the planet for Ceti Alpha VI, sends down Captain Terrell and Chekov, and Khan seizes the pair and then their ship to pursue his revenge on Kirk.