They're an underground brotherhood of evilness. There's seven leaders, and all of them seemed to have known each other at some moment in their past history. They are taking people from Yemeris.
There is an unnamed woman that Finnick says is precious to him, and he is looking for her.
They're linked by an orphanage from the northern reaches. She seems to be hiding, but he's sure she will return to the former location of the orphanage.