The Planet Tartarus is a cursed planet of Khorne under the control of the Maledictum. The stone had a daemon in it which was released by the Chaos Sorcerer Sindri Myr. Sindri ascended to daemon prince for this but was defeated shortly after by Captain Gabriel Angelos.
Dawn of War
Shortly before the first mission of Dawn of War, Chaos Marines of the Alpha Legion, led by Chaos Sorcerer Sindri Myr and Chaos Lord Bale contacted Ork Warboss Orkamungus. They asked the Orks to attack Tartarus to keep the planets forces busy while they searched for an ancient artifact known as The Maledictum. The Orks obliged and each force set to work, intending to betray the other when it was convenient.
Gabriel Angelos and the Blood Ravens third company made planetfall just as the Imperial Guard stationed on the planet were overwhelmed by Orks. Colonel Bromm was one of the few survivors of the Ork assault. Gabriel set about evacuating the civilians as the Blood Ravens pushed the Orks into the wilderness surrounding the cities, allowing the Imperial Guard to concentrate their defenses.
The Blood Ravens continued pursuing the Orks until they made a stand with their Warboss, Orkamungus and his Squiggoth. However both the Squiggoth and Orkamungus were killed in this battle.
After their victory, Inquisitor Mordecai Toth landed on Tartarus. He informed the Blood Ravens that there was a warp storm approaching Tartarus, due to arrive in three days time and that they were to leave the planet immediately. Gabriel refused, stating that he felt there was something amiss on the planet and that he aimed to find out what.
As the Blood Ravens pursued the remaining Orks, they found evidence of other Xenos on the planet. Gabriels suspicions were confirmed when Eldar of the Beil-Tan craftworld, led by Farseer Macha, appeared and ambushed the Blood Ravens. The Orks became a secondary concern as the Eldar continued to attack the Blood Ravens. Meanwhile the Eldar set about delaying the humans so they could reach a buried key which would unleash the power of The Maledictum.
The Blood Ravens and Eldar did battle in the city of Lloovre Marr, where the Eldar summoned The Avatar of Khaine to aid them in their battle. The Avatar was destroyed by the Blood Ravens and they defeated the Eldar in battle. Gabriel and Isador confronted Macha where the key was located. However Sindri stole the key and escaped. Macha told the Blood Ravens that Inquisitor Isador Akios knew of the key, despite denying Chaos being on the planet. Macha also revealed that Chaos forces were massing in the Danaan city sector and left them to fight the Chaos Marines. Macha, having lost the bulk of her forces, withdrew.
The Blood Ravens entered the city and did battle with the Forces of Chaos. They broke into the chapel that Bale and Sindri had barricaded themselves into, however corrupted Imperial Guard covered their escape.
Gabriel called Isador to demand to know his connection with Tartarus and what he knew about the forces of Chaos on the planet. Isador obliged, telling Gabriel that the Maledictum contained a greater daemon of Khorne, sealed away by the Eldar. When the Maledictum was uncovered and blood was spilled on the planet, the daemon called a warp storm to the planet. The storm was intended to corrupt everyone staying on the planet into working with the forces of Chaos, hence Isadors insistence that they leave. However Isador, who was reluctant to follow Gabriel on what he thought was a self destructive crusade, consented to help the Blood Ravens in their fight against Chaos.
It was shortly after this meeting that Sindri, who had been tempting Isador since he made planetfall, finally succeeded in corrupting Isador. Isador decided that the Maledictum was his and that he would use its power for himself.
Sindri and several Chaos Cultists began a ceremony to bring the Maledictum to full power. The ceremony failed and Bale was betrayed by Sindri when the Blood Ravens made it to the site of the ceremony. Isador chose this moment to openly betray the Blood Ravens and he left with the Maledictum, Sindri, the Chaos Marines and Cultists.
Sindri continued his work on a larger ceremony to fully open the Maledictum while he left Isador to deal with the Blood Ravens. After another bloody battle, Gabriel caught up with Isador. Gabriel bested his old friend in battle and killed him.
Sindri, meanwhile, had finally finished his master plan and had ascended to become a Daemon Prince. His glory was short lived; the Blood Ravens defeated Sindri, leaving the Maledictum behind.
Farseer Macha appeared one last time and begged Gabriel not to destroy the stone. However he ignored her and smashed it with his hammer, God Splitter. The Farseer warned him that he'd just damned them all and made quick their escape.
Gabriel was about to leave with the rest of the Blood Ravens when the Greater Daemon appeared before him. The Daemon revealed that it was the one stuck inside the Maledictum. The planet of Tartarus was an ancient altar of the blood god Khorne. All the various deaths that took place on the planet, the Orks, the guardsmen, the marines, the Chaos Marines, Bale, Sindri, Isador, they had all been sacrifices to the Daemon, increasing its power. Had it not been given all these sacrifices, it might have been destroyed along with the stone.
Gabriel had little time to lament his mistake. He warned the Daemon that he was not finished and left with the rest of the Blood Ravens. Shortly thereafter, Tartarus was engulfed in the warp storm.
Tartarus on The Lexicanum