This whole Oathbreaker stuff might just be for the PCs, It's just as likely Ketheric is a classic anti-paladin, or my favorite a Death Knight. Maybe he just traded in his Selunite template for a Sharran one. Being a resurrected hundreds year old Selunite, turned Dark Justiciar, turned tadpole infected cult leader will get you a pass from me for a lot of things.
I don't know enough about Shadow Magic to really comment, but I have to guess that people can use it without supplication to Shar. People have done a lot of theory crafting around the Shadow Magic, but to me it's just one of those hand wavy things you see to explain away super-extra-powerful magic. Why can't a Hag deal with it?...Shadow Magic, bro. How do we stop magic from solving all our problems? More magic.
Well In
A Theory the artifact is the trapped essence of Gith, kept hidden by Vlaakith on Creche K'liir, where a commando squad of Sharran's stole it. Before themselves being abducted by the Mind-Flayers, possibly also after the artifact.
If they were in league with Absolute, I'm not sure why you'd go through that extra step.
