"To win a skirmish requires deadly warriors, the likes of which the oldest legends exalt. To win a battle requires dashing leaders of men, the stuff of grand tales and legend. To win a war requires forethought, planning and grand insight... things that lesser men seldom have a head for."
- —General Derian Arkelius
Derian Arkelius is a General of the Astra Militarum. He is a member of the so-called "Triumvirate" comprised of himself, his younger sister Lord Admiral Anastasia Arkelius and Lord Commissar Vance Theren that collectively commands the Orpheus Salient of the Achilus Crusade.
He is a masterful strategist and an ambitious young officer with dreams of succeeding General Curas. Unfortunately for Arkelius, his capabilities as a field leader are sorely lacking for a man of his station, though he appears unaware of his deficiency in this regard.
As one of General Curas' advisors, his talent for strategic matters came to the fore, and he thrived as a member of the salient's General Staff, devising the strategies that Curas implemented so successfully on Eleusis and many other worlds.
However, his distance from the front lines has, quite at odds with General Curas, made Arkelius callous and uncaring of the soldiers under his command, who he regards more as statistics to be calculated and variables to be accounted for.
As for the Space Marines, to him they simply represent another force that should be under his direct control, and he sees them as nothing but glory seekers who are denying him the chance to prove himself worthy to be Curas' successor.
Almost as soon as news of Curas' death reached him, Arkelius began lobbying Lord Militant Solomon Tetrarchus to replace the late general, sending a barrage of astropathic messages, though none of them appear to have arrived given the lack of response and the smothering presence of the Tyranids' Shadow in the Warp. While his request was never officially denied, neither has he been appointed commander of the Orpheus Salient outright.
This is why, when Lord Commissar Vance Theren proposed his deception of avoiding any mention of Curas' death to the troops under their command, General Arkelius jumped at the chance. If nothing else, it would give him an opportunity to deny his sister the ability to make a stronger case for herself. His attitude towards the Deathwatch (and the Adeptus Astartes in general) is barely constrained hostility.
Arkelius' ambition, and the callousness with which he regards the troops under his command, are a dangerous combination in the face of the Tyranids, and more than once he has clashed with Lord Commissar Vance Theren over so reckless use of the increasingly limited forces available.
In the five Terran years since the Triumvirate was formed, Derian Arkelius has become a shadow of his former self. His rivalry with his sister -- always fierce -- has grown in intensity, as control of the salient's forces is now at stake, and the increasing pressure of commanding forces in the face of the Tyranid advance has taken a toll.
Arkelius now seems far older than his years alone would suggest, and his mind is fraying, leaving little more than cynical ambition unrestrained by wisdom or caution, and an increasing sense of enmity towards any who he feels is a challenge or rival to his authority.
The worst of this hostility has not yet started to take a toll on the salient's forces. He is continually protected by an honour guard drawn from the veteran squads of his personal regiment, the Scintillan 84th Grenadiers, and his orders are vetted by Lord Commissar Theren and filtered through many layers of subordinate commanders.
As yet, only the higher levels of the salient's command have heard more than rumours about the general's worsening mental state, and few beyond Lord Admiral Arkelius and Lord Commissar Theren know how bad things are.
However, Arkelius has begun to make enemies, his decisions coloured by greed and ambition. Lord Castellan Ortarna Lokk of Castobel is a particular source of consternation, being well loved by his men and a decisive and successful commander, while Steward Militant Vierling of Hethgard is regarded as a rival far closer to home, an officer extremely similar to Arkelius in talent and ambition.
Worse still, Arkelius has managed to alienate the Astartes as well, regarding them as a dangerous and unpredictable element, a variable he cannot control or account for, and this distrust and resentment has hindered collaboration between the Astra Militarum and the Adeptus Astartes on many worlds.
This is most evident on Hethgard, where Arkelius has repeatedly clashed with Captain Abroghan Callister, the commander of the Storm Wardens presence on the world, whose pride and temper do not allow him to tolerate "throne-room strategists," as he calls those who command armies without braving the front lines.
He has also found enemies amongst the Deathwatch, who he regards as even more unpredictable than most Astartes, unbound as they are to the Achilus Crusade, and there is no love lost between General Arkelius and the few Kill-teams who have had the misfortune of dealing with him.
Now that the war has reached his command post on Hethgard, he has no choice but to deal directly with the troops under his command, rather than to orchestrate their movement from afar as is his preference.
A poor leader, incapable of relating to his troops, and ill-suited to command in the heat of battle, Arkelius cannot easily bring to bear the strategic mind that won him such renown, and the world's defence can ill-afford to be in the hands of a commander so poorly suited to the situation.
- Deathwatch: The Achilus Assault (RPG), pg. 142
- Deathwatch: The Jericho Reach (RPG, pg. 135