The Flying Mongeese
Sinruth wants to raise an army and carve out a fiefdom of his own, but to do so, he knew that he needed a place for his command center, so he went looking for one. When he found the catacombs of Castle Rivenroar, Sinruth decided that they could serve as a starting point—and perhaps expand as needed later. So, he set to work clearing it out and making alliances when he found a need for them.
The castle itself and its catacombs housed the Rivenroar family (living and deceased) long ago. Some other local families of note used the catacombs as a place of interment as well. These families and the Rivenroars had more than one thing in common, but now only those who know what they’re looking at as they journey through the catacombs can figure it out: a reverence for Vecna. Now the castle and the catacombs house a much different set of inhabitants than those who built it long ago might have envisioned.