I'm not sure what they plan to do with Karness Muur. I love the idea of new ancient sith lords introduced but the guy is carrying a lightsaber and is said to have been one of the ancients. That doesn't make much sense since the ancients had no contact with the republic and weren't using lightsabers..
According to a massive amount of retconning and the newish Jedi vs. Sith Guide to the Force, lightsabers were introduced before the Great Schism and were carried by the Dark Jedi Exiles. It is proposed that they fell out of use due to lack of supplies to build them with, lack of energy cells to power them with, or simply because they just stopped using them in favor of their creations.
No I know this Rex but "Old sith Empire" refers to the Empire the exiles created. They did not have any weapons when they came to Korriban and it was stated that it was highly unlikely they went back to sabers seeing as how they needed power packs and instead switched to swords. Of course this would retcon Tulak Hord completely.. Ah the Star Wars inconsistencies continue.
Speculation leads me to believe that Carrick could be nihilus. this would be the most logical assumption if he is to be anyone significant. He couldn't be the exile; the exile is female, and exiled after teh mandalorian wars. He could be an insignificant jedi padawan with an interesting character and story more likely than not.
Remember also that skywalker is the force child of griegor the wise who generated him using his midichlorian creating abilities. Force strength would be considerably weak before Anakin in the family tree.