I'll likely be doing this if nobody in this thread argues me around. Loyalist Abyssals bore me and redemptionist Abyssals do better as player characters, and nobody wants to play one.
The other option is to use
@EarthScorpion's rewrite of the deathlords, and have Abyssals as the daywalker servants/allies/pawns/slaves/students/lovers/etc. to the corrupted pseudo-Unquestionable lords, ladies, and abominations which dwell in the depths of the world. Any one Abyssal is likely to differentiate massively from another, because
who they're working for,
why, and
to what end ends up defining large swathes of their behavior. They're the envoys of the deep deep dead, the Stygian realm that could previously be ignored by the living; their masters push them to seek out (or destroy) ancient knowledge, fulfill grudges unsatisfied since the Great Contagion, resurrect lost traditions, and/or retrieve people, goods, or entire kingdoms so they may be brought away into the night, never to see the sun again.
One Abyssal is a terrified former merchant who goes around collecting gold and jewels to placate the greed of a giant dragon made of corpses that lives in one of the Rivers of Death. Another is a starry-eyed, steely-gazed hardcore optimist who hires sages and scholars to assist the Duchess' work with the Well of Udr, so that all may one day prosper and the world be healed. A third is a woman who died during the Trials of Sorcery, and now seeks to prove her brilliance by helping Ten Thousand Sublime Calculations Embodied As Stars reshape the Dome of the Heavens and correct the hideous disharmony of its constellations. They can serve as allies, enemies, or whatever else you need depending on who they are and what they're working for.
As for their powers, there's been previous ideas of having their Charms key off of mythology about vampires, ghouls, zombies, ghosts, and other death-related creatures, so you'd have a cluster of Charms to let you be Jason Voorhees, a cluster of Charms to let you be an Apostle from
Berserk, a cluster of Charms to let you be Judge Death, a cluster of Charms to let you be Ghost Rider, and a cluster of Charms to let you be Samara from
Ringu.
Another idea I've seen was for them make a pseudo-Devil Domain by absorbing chunks of the Underworld to forge their own personal Labyrinth, which they hold within themselves - and extrude into their environment to create environmental hazards, alter terrain, and otherwise forcibly grant themselves a home field advantage.