Grialdyn is a monster found at Servant Quarters ET. It is usually invisible to everyone but the Castle Master, but can be made visible (night) or invisible(day) by interacting with the Newerthian Clock at Tower D. This only applies to Grialdyn itself and not the Grialdyn Cores, five smaller monsters who surround it. It wishes to empower itself with darkness, claiming that darkness belongs to it and it alone.
For the battle to start, the player must activate a terminal at the top of the room. Fortuanately for the player, there are several hearts at the far left of the room which helps if the player runs low on HP. Once Grialdyn is defeated, it'll call out to it's "father" before dying. Shinica pops in, unable to understand what Grialdyn is talking about before showing amazement that Serenity manged to defeat the ancient security system. After it's death, you can choose to heal a burned Shinica with the Life Pot.
Emerald Flame S (Griadyn Cores fire a large amount of them out, but they don't deal much damage.)
Emerald Flame L (Graildyn doesn't use this as often, but often mixes them with Emerald Flame S)
4-Cutter (Graildyn uses this attack occasionally, and can be easily dodged. It deals 20 damage if it connects so be careful)
North Wind (Graildyn calls upon a gust of wind to blow back Serenity, is used in tandem with other attacks)
- Grialdyn is the first boss that can be battled whether it is invisible or visible. It's also one of the few bosses that has several hearts in it's arena, and for it's cores to drop hearts.
- Grialdyn uses the AI of the Core from Cave Story, being a white palette swap of it.
- Unlike Sycor, it's cores also take damage and can be pushed back. It's also possible to temporarily disable them.