Casan Sabius is the current Lord Regent of the Red Scorpions chapter, a position roughly equivalent to a provisional Lord Commander. He assumed command after Carab Culln fell in combat during the Indomnitus Crusade against a Tyrannid bioform known as "The great Beast of Sarum", and was believed lost to the Imperium. Like his predecessor, Sabius has a thing for leading his Chapter from the front lines, seeking to slay the mightiest foes in order to inspire his fellow Scorpions by example. As far as his battle-brothers are concerned, he is considered a formidable strategist and swordsman (having already saved the Chapter from Hive Fleet Kraken on one occasion), and is a worthy successor to Culln.
But while his brothers are happy with having him in charge, Sabius himself is not. He secretly blames himself for the loss of Culln, and his zeal and fury are said to be caused by a desire to atone for his mistakes. This feeling is rooted so deeply inside him that Sabius believes his role as leader of the Scorpions to be nothing more than a stage of transition for the Chapter until a new Lord Commander can be appointed. As anyone slightly familiar with Red Scorpion behavior might expect, all of this pushes the Lord Regent to strive for genetic purity in a quest that is borderline fanatical even by his Chapter's already insane standards. In battle he is often accompanied by Sirae Karagon, the Chapter's guardian of purity and standard bearer of the Vexilla Imperialis, who acts as a close advisor and pragmatic foil to Sabius's fervor.
Model-wise he perfectly embodies the theme of his Chapter: a thick layer of generic iconography resting upon far more ancient, which ends up making him look subtly distinct from it's vanilla marine counterparts. His artificer armor seems to be mostly MKIV pattern, but has elements of Corvus armor mixed here and there, and that bog-standard bolt pistol of his is also heresy-pattern for some reason.
On the Tabletop
Sabius is the Red Scorpions Chapter Master in all but name: he has the statline, and the standard 6" re-roll to hit aura that other Chapter Masters have. His unique rule "Purity of Aspect, Action and Intent" gives all the models of a RED SCORPIONS unit within 6" an additional attack as long as Casan Sabius himself manages to wound something in that fight phase first. Lead by example. As for wargear, he has artificer armor, an iron halo, frag and krak grenades, the Blade of the Scorpion which is a +1S D3D relic power sword that doubles it's strength value when targetting a MONSTER or VEHICLE, and a bolt pistol.
Sabius is the last non-Primaris marine release to have received rules for 8th edition.