Decius Achare was the son of Samatheon Achare, the head of one of the Bannor's most prominent houses. He was recalled from the front at their border with the Clan to return home to defend Torrolerial, at the diviner's wishes. He was the commander to whom Valin Phanuel and Rosier answered when they fought the Burnt Priest and the Twisted men, and when they traced the taint of the priest to the cult of the Overlords and were deployed to crush the cult where it started, in the lands of the Lanun. Rosier's faith wavered in these missions, and he defied the Order's priests when he used the magic mask of an OO Speaker to walk on water so as to return to Cuantine (a suburb of Torrolerial) quickly enough to defend it. Decius is given a choice of whether to demand he return and be punished (in which case he will run away and hold an eternal grudge) or to agree that there was no sin there but tell him that he must leave because the priests won't agree.
After crushing the cult, Decius returns to his father's house and has a nightmare in which he is attacked by a talking snake. He dives under his bed and grabs the snake, and then wakes up to find that he is instead holding a glowing Octopus Idol. His father believes that he has given into temptation by a demon and now worships the Overlords, so he calls for a diviner and a trial (which wouldn't probably lead to his son's execution). (Later, it is revealed that Rosier's uncle was a necromancer who used dark magic to frame Decius.)
Decius decides to flee across the sea to the desert of Myrh to avoid trial. He can choose to join the Malakim if he wants to maintain his honor despite being framed, or to the Calabim if he has had enough with such laws.
If he chooses the Calabim, Flauros has him put down a settlement of Hill Giants that has been troubling his lands. He gives Decius command over several actual vampires, much to the nobility's chagrin. He succeeds, but gets a bigger mission when he uncovers a priest of Lugus there. Flauros is surprised to see that first priest of Lugus in this desert in 600 years, and afraid as the god knows his secret and has a grudge against his kind. He orders Decius to put down the faith, but to do it subtly so that it cannot be traced to them. As such, he introduces him to the Council of Esus to learn their assassin's secrets. Eventually Decius defies his orders and insists they must declare open war to stop the faith. Flauros tries to kill him, but decides not to when Decius threatens that he would surely land one blow with his poisoned blade first which could cause Flauros great pain for all his eternal life. Decius admires his spunk, and agrees. The Undercouncil (which presumably had long existed before this) declares open war on the Overcouncil, and the Sheaim manage to summon the Infernals. While Flauros and Alexis want only more victims for their ritual, Decius demands all the lands around Tempus Mor, enough to him an empire large enough to rival the Bannor. He runs into Valin and Rosier again. He captures Talia Gossam, and keeps her prisoner instead of letting the vamp who captured her feed. He is surprisingly kind to her, and says he will let her go when her husband asks for her. That takes a while, and in the mean time she forgets she was a prisoner. Os-Gabella decides to get the rest of the Undercouncil to isolate Decius, to drive him and Talia closer and encourage her to have an affair to hurt Varn. Talia and Decius do fall for one another, and have sex at least once shortly before Varn finally asks for her. On the way back, Os-Gabella appears to her (in disguise) and tells the tale of how a good and honorable king (Kylorin, I presume) and his empire (Patria) fell because of the infidelity of his wife. She then calls on the demoness Lethe to tempt Talia into committing suicide. Talia returns home and tells Varn everything. He forgives her, but is too distraught to fulfill his religious and political duties, which he gives to Chalid. Eventually, Lethe succeeds and Talia kills herself, and when Varn learns of Lethe he goes on a fools errand to seek revenge by defeating the demoness and purging the Fane of the Lessers to of the demons who had welt there since the godswar. In the meantime, the Sidar try to assassinate all the leaders of both councils. Decius kills the assassin sent for him and Flauros, but refuses Flauros's dark gift. Flauros leaves in disgust, but lets him keep his empire. Os-Gabella commissions him to find the Nether Blade (which had not only killed Ethne but trapped her soul in the Netherworld) in the hopes that it could be used to bring her true death, saying her could name any price for it. Decius invades the Sidar lands, and faces some remaining Malakim there as well. ON the way, a Succubus sent by Os-Gabella convinces him to make a pact with the evil gods and he becomes evil instead of neutral. Eventually he corners Sandalphon and demands to know why the sorceress wants the blade. He convinces him to give in lest he continue using necromancy on the Sidar corpses. He learns what it does, but decides to keep it instead of getting the bounty. The Sidar inform him of Auric's imminent ascension, and he sails off to stop it. The Godslayer cannot be found, but he figures the Nether Blade might work too. He meets his father as he arrives to join the Bannor, Lanun, and Hippus in their quest, but the meeting is not pleasant. His father is ashamed to see the monster he has become, even though he admits that they found out that he had been framed before. The closest thing to good words he has to say to his son is that he hopes he dies honorably there in battle instead of him needing to put his son on trial and executing him himself afterwards. We don't really know what happens to him, but e do know that Rhoanna and Falamar get the blade in time to vanquish Auric to the Netherworld (where he and Ethne presumably join Laroth's army).
If he chooses to join the Malakim, he quickly becomes a general whose military prowess greatly aids Varn Gossam in spreading his the new faith of the Empyrean (although he personally does not convert) and in founding the Overcouncil. He met Talia Gossam before meeting Varn, and there was a mutual attraction that combined with Varn's neglecting her for his religious and diplomatic duties led to a sexual affair. I don't think she kills herself this way, but she might still. The Overcouncil wins the war, but more demons emerge from new passages all over the world and the Mercurians' war against them causes serious collateral damage to bystanders. The Sidar assassins still come for all the leaders. Decius captures his for questioning, while Ethne's assassin (Rathaus Denmora) still steals her souls with the Netherblade. On the way, Decius prays to all good gods for forgiveness for the affair and other past sins, and promises to be a better man in the future (thus changing his algnment to good). An Elohim archmage begs Decius to go find the blade, so that they can destroy it and allow Ethne to be resurrected. He agrees, but when he finds Sandalphon is informed of the threat from Auric. He decides that the Nether Blade is the only hope to save Erebus from another Age of Ice, and so makes the decision to damn Ethne to the Netherworld to save the world of the living. He meets his father on the way to fight Auric, and their meeting is much more friendly. They don't spend much time together then, but his father promises that if they both survive that war he would be welcomed home and that the Bannor would consider joining the Overcouncil and possibly even tolerating some reforms that allow then to show a little mercy.
