This now needs revision.
With Sephridus swearing fealty to the Shadovar, the Shadovar faction and the Candlekeep faction have merged. With the new power behind them, you may have noticed in-game that there's less confusion about their goals and that the characters are more open about what they intend to do. It is now possible to apply to play in the Shadovar directly.
The merger is much the same as the Red Hart/Court merger. Shadovar, Candlekeep, and New Kilgrave are the same faction, ultimately reporting the the same NPC (Lord Chancellor Sephridus) and working on the same goal. However, they have distinct methods of working on that goal. The Shadovar will spend their time exploring the Helmlands, looking for artifacts of great power and participating in subtle politics while Custodians scour the ruins and participate in the public polity of the city.
Right now, it's the fullest faction we have. Good enough concepts can still get in, but I know that I'm personally leery of exceeding the six person cap when we have other factions so empty. As exciting as it might be to play one of these characters, please seriously consider CoG, the Sheriffs, and Court Heralds.