Sorrow, or the Vernacht, is a powerful creature that's sealed under the castle. It's from the castle's world, but it's about as powerful as anything else in this world, from what I've heard. The Lord of the castle thinks he can use its power, so he weakens the seal on it, letting it corrupt people and creatures. But I suspect it's more using him than he's using it.

[It's a very short version of what the Sorrow is, but it explains enough]

And I hope they aren't, too...they're endangering people they have no right to.

[Sync glances down, feeling strange at that thought. Almost afraid. Why would he be afraid? But he can't afford to listen to that feeling anyway]