Exegol is a dark desert planet located within the Unknown Regions and is a hidden Sith world.
Exegol is a dark, barren, and rocky world with desert flats. The planet's dry conditions combined with the rubbing of dust particles in it's atmosphere created enormous static discharges that appeared as lightning strikes on the surface.
Exegol (previously called Ixigul) used to be a fertile planet before the Sith established themselves in there and stripped it of all of its resources, leaving the world barren. The Sith loyalists suspected the existence of a transportative vergence beneath the planet's surface, so they excavated the enormous fissures on its crust trying to find it.
According to legend, Exegol was the hidden world of the Sith because it did not appear any starcharts of the galaxy like other Sith planets, such as Malachor or Moraband. However, it was said that the only way to get to Exegol was through the use of a Sith wayfinder, an artifact created by the Sith to be used as a compass to provide safe passage to Exegol.
After Kylo Ren found one of the wayfinders on Mustafar, he travels to Exegol and meets a revived Darth Sidious who offers him the throne of the Sith and the command the Final Order, a massive fleet of Xyston-class Star Destroyers, if he kills Rey. Meanwhile, Rey and other members of the Resistance look for the second wayfinder, but Ren destroys the wayfinder in front of her in the ruins of the second Death Star.
Rey duels Kylo, but a dying Leia reaches for them, ending their fight. Rey saves his life and confesses her feelings for him which helps to redeem Kylo Ren, along with his mother's sacrifice. Rey then leaves aboard his TIE whisper, which has the first wayfinder inside. Rey travels to Ahch-To to exile herself and burns Ren's ship, unaware of the wayfinder inside it. But after being convinced by Luke's Force spirit to face Sidious, Rey finds and recovers Ren's wayfinder from the wreckage of his TIE whisper and uses it to guide the Resistance to Exegol. Ben Solo finds an old TIE scout in the wreckage of the second Death Star and drives it to Exegol while following the tracking signals Rey is transmitting.
The battle of the Resistance against the Sith Eternal forces, results in the Resistance's victory, while Rey manages to defeat Sidious and the Sith with the help of Ben Solo and the Jedi from the past.