Hell was a relatively primitive world populated by the passive Helkans. The Helkans were largely only interested in trading the planet's only export, helkogen gas, which existed in abundance and was cheap to buy, but it was also extremely deadly.
"This war is getting dirtier"
During the Second Dalek War, the Daleks appeared in the skies of Hell and invaded the planet before the Helkans could react. They were enslaved and some of them were set to work mining more helkogen gas. Above the planet, the Daleks constructed the Death Wheel, a huge space station and doomsday weapon to which the gas was transferred in cargo ships.
Once the required amount of gas had been reached, the Death Wheel was capable of releasing it on the planet below, assuring the extinction of the Helkans. The Daleks believed the success of such a weapon could help them achieve universal domination. The Emperor Dalek himself oversaw the completion of the station.
Many of the Helkans were worked to death in order for the Daleks to reach their goal. Other survivors began asking for food and sleep. When this was reported by an Ogron enforcer, the Emperor exterminated him. Similar reports from other Daleks led the Emperor to declare that slaves deemed weak would be executed.
As the Death Wheel neared completion, Abslom Daak and his crew aboard the Kill-Wagon arrived above the planet and engaged the Dalek defenders. The Daleks issued a red alert and attacked the Kill-Wagon, which was shot down and crashed on Hell's surface. Daak escaped unscathed but he assumed the rest of the crew - Prince Salander, Vol Mercurius and Harma - had perished. The Daleks also believed this and called off the red alert.
The Seventh Doctor arrived in the TARDIS around the same time and, in his ignorance, was almost exterminated by the Daleks while exploring, only to be saved by Daak. The two attacked one of the areas in which the gas was being mined and destroyed the Dalek guards. The Helkans were freed but initially appeared as if their spirit had been broken.
One Helkan, Kelmo, was comforted by his lover, but she was sudden killed from behind by a surviving Dalek. Kelmo flew into a rage and killed the Dalek in retaliation. The Helkans then joined the Doctor and Daak on an assault on the source of the Daleks' operations.
Death Wheel infiltrated
The Doctor and his party took control of cargo vessel K-6 and flew up to the Death Wheel but the attempt at stealth was unsuccessful as the occupants were unable to transmit the proper security codes. Upon docking with the Death Wheel, the Doctor and the Helkans were taken prisoner, although Daak remained at liberty and conducted his own reconnaissance mission.
The Doctor was interrogated by the Emperor and the Imperial Guard, who verified that he was the Doctor based on his memories of his previous incarnations. The Emperor ordered the Imperial Guard to kill him but Daak barged into the room and cut the Guards up with his chainsaw, though the Emperor escaped with little damage.
"Only one option left"
Free from captivity, the Doctor and the surviving party members had only hours left to prevent the Death Wheel from deploying the gas. The Doctor soberly announced that it may be necessary for all of them to sacrifice their lives. Appropriating a Dalek hoverbout, Daak brought the party towards the central reactor. The Doctor instructed the others to flee the Death Wheel while he sacrificed himself to destroy it. (COMIC: Nemesis of the Daleks)
However, Daak, who had held a violent personal vendetta against the Daleks ever since the Dalek invasion of Mazam, (COMIC: Abslom Daak... Dalek Killer) knocked the Doctor out and took his place. He rammed the hoverbout into the reactor core, sending it critical and destroying the station. Daak saw this as a fulfilment of his promise to his dead lover, Princess Taiyin, to "kill every damned stinking Dalek in the galaxy!"
Aftermath and future intervention
Although Daak and the Doctor both believed the rest of the crew of the Kill-Wagon to be dead, they had in fact survived and were nursed back to health by the Helkans.
Daak himself also survived due to Dalek interference from the future, beyond the period of the Second Dalek War. The Daleks involved were on the brink of civil war as Davros continued to gather support. The Emperor sought to capture the Doctor so he could lead them to Davros, and decided that Daak could be used to capture the Doctor.
At the instant before his death, Daak was extracted via transmat and recruited by Daleks - disguised as members of the tribunal which sentenced him to exile - to hunt down the Doctor. Daak returned to Hell in the aftermath of Operation Genocide, where he was reunited with his crew and captured the Doctor, now travelling in the company of Bernice Summerfield. Once the Doctor had been located, the Daleks scooped everyone out of their own time and across space, transporting them to Skaro. (COMIC: Emperor of the Daleks!)