Vashtorr

Vashtorr the Arkifane is a demon of (thus far) indeterminate allegiance whose technological skills have made him not only a figure of adoration among the Dark Mechanicus and Warpsmiths among the dark powers, but also the mastermind behind the newfangled Arks of Omen, weaponized Space Hulks that are part of Abaddon's renewed offensive on the Nachmund Gauntlet.

Thus far, the only real opposition he faces is from Be'lakor, who it turns out not only to suffer from abandonment issues from his four parents, but also from potential son figures as well, as Big B used to be the big secret confidant of the Warmaster and was just as duplicitous.