Unknown, once in the tens of millions
Tartarus was an Imperial Civilised World besieged by Orks, and at one point, five different factions clashed on the planet: they included the Orks, the regiments of the Imperial Guard, the Chaos Space Marines of the Alpha Legion, the Eldar of the Biel-tan Craftworld, and the Space Marines of the Blood Ravens Chapter. The world was the setting for the Dawn of War PC game.
At the very end of their campaign on Tartarus, the Blood Ravens find out that there had been a very early human colony settled on Tartarus roughly around the time of the Great Crusade ten thousand years before. However, a Greater Daemon of Khorne that cyclically emerged from the Warp every three thousand years had appeared on the Tartaran colony and seduced the whole population into the worship of Chaos. In response to this, Farseer Macha of the Biel Tan Craftworld had arrived with an Eldar army to stop this threat. Macha held that it was her duty to defeat and trap the daemon, and would later suppress his will every 3,000 years. In the ensuing fight, the Eldar Guardians had the task of holding back the Chaos Cultist forces and Khornate Bloodletter daemons, as Macha held a Warp Storm at bay and the Biel Tan's Avatar of Khaine fought the Greater Daemon of Khorne. The Eldar were victorious, but the cost of victory was the loss of their Avatar. Macha trapped the daemon in a stone called the Maledictum, then buried it beneath the surface of Tartarus. To be certain that the stone could not fall into the wrong hands, the location of its burial could only be learned through the solution of a series of riddles and clues.
At around the same time as this first incident, a kill-team of Deathwatch Space Marines accompanying an Ordo Xenos Inquisitor named Jhordine had been tasked with finding the Avatar's weapon, a potent Eldar sword called the Wailing Doom. They succeed in finding it, though the weapon had been shattered in the battle with the Greater Daemon, and a Blood Ravens Captain attached to the Deathwatch kill-team managed to recover a shard of the Wailing Doom. After the team escaped, the shard would be used in the construction of the Daemonhammer Godsplitter. All the records of this event were later expunged by the Inquisition. Tartarus would be recolonized by the Imperium 300 years later and treated as a new planet that had never before been inhabited by humans.
Over the centuries, the new Imperial colony on Tartarus defended itself, and successfully fended off many invasions by the Orks and pirates of its region of space until 999.M41. The Greater Daemon trapped in the Maledictum had began to influence the Tartaran population once again, inspiring fear and heresy among the people of the world, including the members of the Tartaran Imperial Guard Regiments under the command of Colonel Brom and the Tartarus Planetary Defence Forces. Simultaneously, warbands from the Alpha Legion of Chaos Space Marines, lead by the Chaos Sorcerer Sindri Myr and the Chaos Lord Bale, tried to take advantage of the reawakening of the Maledictum, and encouraged a horde of Orks to launch a WAAAGH! against the planet and serve as a distraction while they uncovered the resting place of the Maledictum. With the defense of the planet at the forefront of the Imperial forces' attentions, the Alpha Legion's actions in search of the Maledictum were largely unknown until the Blood Ravens uncovered evidence that linked the ongoing growth of Chaos activity on Tartarus with the recent invasion of the Orks.
It was during the failing defence of a key Tartaran spaceport by the forces of the Imperial Guard that the Third and Fourth Companies of the Blood Ravens arrived on-planet and started to force the Orks back. Lead by the Blood Ravens Force Commander, Captain Gabriel Angelos of the Third Company, who was accompanied by his long-time friend Librarian Isador Akios, the Blood Ravens helped hold back the green tide of Orks. In the aftermath of one battle, Angelos was confronted by Inquisitor Mordecai Toth, who urged him to take the Blood Ravens and leave Tartarus before an incoming Warp Storm arrived. Determined to root out all traces of Chaos from the world, Captain Angelos refused and continued the search for heretics, and later the Maledictum as he learned more about the artefact left behind by the Eldar. In due time, the Blood Ravens would soon come into conflict with the Biel-Tan Eldar, again lead by Farseer Macha, who had been working to keep the Maledictum hidden from Imperials and the forces of Chaos alike. During a brief confrontation between Captain Angelos and Farseer Macha, the Chaos Sorcerer Sindri stole the Maledictum. The Blood Ravens soon attacked the Alpha Legion, who were attempting to free the Maledictum from its protective bindings. As part of a scheme by Sindri, he escaped, letting Angelos kill the Alpha Legion's Chaos Lord Bale; then the Blood Ravens Librarian Akios, who had just turned to Chaos with Sindri's help in a foolish attempt to use the powers of Chaos against the forces of Chaos, stole the Maledictum and fled, only to have the Maledictum later taken away by Sindri, and be used by the sorcerer to distract the Blood Ravens as Sindri worked to unleash the power of the Maledictum. In a duel, Angelos defeated and killed his oldest friend Akios, making the Librarian an example of what happened to all those who became traitors and heretics.
Afterwards, as the Blood Ravens moved to attack the remaining forces of the Alpha Legion on Tartarus, Toth apologized to Angelos, as he had at first sensed the taint of Chaos near him. Although Akios had actually been the heretic, Toth had been nearly certain that Angelos was the true Traitor, largely due to Angelos' prior actions on the world of Cyrene, where he had called down an Exterminatus order on his own homeworld. Toth then entrusted Angelos with his personal weapon, which Toth revealed to be the Daemonhammer Godsplitter that had been forged from the weapon used by the Avatar of Khaine that had previously defeated the Greater Daemon that now took up residence in the Maledictum. In the meantime, the sorcerer Sindri had unlocked the potent powers of Chaos bound into the Maledictum, so that his body would ascend to become a Daemon Prince of Chaos Undivided by a truncated process of daemonic ascension. Aided by Inquisitor Toth and the Farseer Macha's remaining Eldar forces, the Blood Ravens pushed back the Alpha Legion and killed Sindri in his Daemon Prince form.
Against Farseer Macha's desperate warnings, Captain Angelos used Godsplitter to destroy the Maledictum, believing that the destruction of the unholy artefact would destroy the Greater Daemon within. Farseer Macha proclaimed that Angelos had "just damned us all," and swiftly departed. As Toth and the rest of the Blood Ravens begin to flee the oncoming Warp storm, Angelos paused and was confronted by the Greater Daemon, now free from its imprisonment within the Maledictum. The Daemon revealed to Angelos that Tartarus had served as nothing more than a massive planetary altar dedicated to Chaos, and that every death since the Orks had invaded the world was a sacrifice to it to strengthen it and allow it to escape its confinement. Having unwittingly freed the Greater Daemon and provided it with the best sacrifices to feed its hunger, such as the deaths of Isador Akios and Syndri Myr, the Greater Daemon allowed Angelos to escape with his life--but promised that they would meet again one day. Even as Angelos left the doomed world behind to be swallowed in the massive Warp Storm that had been called by the daemon's presence on the world, vowed to hunt down and destroy the terrible evil he had inadvertently released into the universe.
By this point, the civilian population of Tartarus had largely evaporated due to the assault of the Ork WAAAGH!, and the rest had already fled the planet before the oncoming Warp Storm; the cities of Tartarus had been destroyed while the large concentration of Chaos forces and cultists on the surface had greatly warped the landscape around them. As the Imperial forces jumped into Warpspace, Tartarus was consumed by the Warp Storm and lost to the mortal realm.
Terrain and Climate
The planet of Tartarus was made up of several well-populated cities dispersed among vast forests, and had at least one major river. All the water had been pumped out of one flood valley, so that it could be used as the site of the planet's capital city, Lloovre Marr. Later on, when the Blood Ravens Space Marines arrived on the world, they note that the land around Lloovre Marr has been saturated with enough blood to be turned into a bog.
- Warhammer 40,000: Dawn of War (PC Game)
- Dawn of War (Novel) by CS Goto