Given Basic Information By Persephone
The raid on Hades was when one of the demigods in the Titan Army entered the Underworld and stole The Sword of Hades (an unauthorized symbol of power that Persephone ordered to be created). The demigod later revealed as Ethan Nakamura was sent to steal the sword for Kronos because it had one of The Keys of Hades (which would be able to imprison a soul or release one). Persephone abducted Percy Jackson, Thalia Grace and Nico di Angelo to recover the sword. They were given a plant which they were told would lead them to the sword and as the closer the thief got towards escaping petals would begin to fall off.
Encounter With Sisyphus In The Fields of Punishment
The plant led them to The Fields of Asphodel then to The Fields of Punishment where they came across Sisyphus. Thalia agreed to push his boulder for a while in exchange for information. Sisyphus informed them the demigod had one eye a package (the sword) and that he had told the demigod to see Melione. This information let the three know who the thief was. Sisyphus then tried to distract the three as to escape and permanently leave Thalia with the boulder (this attempt failed).
Fight With The Keres Further Knowledge About The Sword
The flower led them through volcanic rock hills where they encountered the Keres (spirits of violent battle, disease and pestelence). Nico commanded them as the son of Hades to stay back. The Keres said they had a new master and that Hades would soon be defeated. The Keres then attacked them. One of them cut Percy's shoulder (leaving a wound that could only be cured by a god or titan). One of the Keres said "Iapetus shall crush you!" It was because of this encounter that the three were able to deduce that Ethan had been recruiting allies in Hades and that the sword also had the ability to summon the damned from Tartarus but that Ethan had already used it to release one of the Titans who had not yet escaped as Kronos, Krios and Hyperion had which was established in The Battle of The Labyrinth.
Crossing The River Lethe
Along the way Nico informed Percy and Thalia that Melione was the goddess of ghosts and she had her own way out of Hades, this meant if Ethan made a deal then he would have an escape route. The flower led them to The River Lethe and while Nico and Thalia were discussing tactics to get across despite being wounded bad enough that he could barely stand Percy manipulated the river forming an arch so they could cross (this showing his power extended even towards the rivers in the Underworld). Percy made it halfway across before his concentration broke and it fell, just before it hit him Percy thought "dry" thus avoiding the Lethe's power. Once he got out Percy passed out but was revived by nectar.
Melione & The Mountain That Led To Her Cave
They next had to follow a path lined with human bones up a mountain that was a few hundred feet high. They were however detained by Melione whose ability to form the shape of ghosts scared Thalia and Nico. Melione took the shape of their respective mothers. Melione was however unable to find any ghosts Percy had which allowed him to fight her off. It was at this point the last petal fell.
Fight With Iapetus and Ethan Nakamura And Recovery Of The Sword of Hades
As fate would have it, the trio had gotten close enough that despite being at the mouth of the cave and about to escape to the surface, when Iapetus armed with his spear saw the three demigods he turned to attack them despite the protest of Ethan. Iapetus accidentally knocked Ethan down causing Ethan to drop the Sword of Hades. Iapetus charged with great speed at Percy barely missing and instead impaling a rock. Thalia tried to provide aid by shooting Iapetus everywhere from his shoulder to knee. This however annoyed Iapetus more than it hurt him. Ethan attempted to provide aid but Nico summoned three skeleton warriors to occupy Ethan. Iapetus engaged Thalia who also barely dodged his attack. She dropped her bow and pulled her hunting knives though at close range against Iapeus's spear would not help her much. Despite the enormous pain Percy saved her by lunging at Iapetus and impaling his calf. Iapetus responded by hitting Percy with the shaft of his spear sending him flying and crashing into the rocks at the banks of the Lethe. Iapetus hobbled towards Percy and despite Thalia and Nico's attempts to distract him Iapetus kept going towards Percy. Percy at the banks of the Lethe came up with an idea, he insulted Iapetus who attacked with his spear. Percy sidestepped Iapetus' spear again and Iapetus impaled the ground. Percy grabbed Iapetus by the collar, the two grappled and fell into the Lethe. Despite the pain Percy's aquatic power kept his memory safe, Iapetus was not so lucky. Percy emerged from the river dragging Iapetus. Up at the cave Ethan was finishing off the skeletons when he looked down and realized what had happened to Iapetus who asked Percy "who am I?" Percy replied "you're my friend," "you're...Bob." While Iapetus was no longer a threat Ethan saw the sword and was going to lunge for the sword when Thalia shot an arrow. Ethan realized his raid had failed so he used Melione's cave to escape. With no threat, Nico was able to with ease pick up the sword. Bob meanwhile had healed Percy of the wound received by the Keres.