Grialdyn is a monster found at Servant Quarters ET. It is usually invisible to everyone but the Castle Master, but can be made visible or invisible by interacting with the Newerthian Clock at Tower D.
Grialdyn Cores are five small monsters surrounding Grialdyn.
- Grialdyn is the first boss that can be battled whether it is invisible or visible.
- Grialdyn uses the AI of the Core from Cave Story.