Kassias Grandiel is the boss of the Wizard's Labyrinth and wields a clone of the Klara Libri. Kassias Grandiel was once one of the most powerful sorcerers of Zeruel and served as an adviser to one of the earliest Elven kings.
In the past, Kassias Grandiel was somehow in possession of the clone of the Klara Libri and was an old friend of Baldinar, but noticed a black aura enveloping him. Thanks to his suspicions, he managed to escape the explosion that annihilated Kounat.
Grandiel witnessed the surviving elves decaying into savagery while engaging the dwarves in war, certain that they were responsible for the death of the Elven King. However, he never ceased to think that in fact, it was Baldinar who was responsible. Disgusted by the increasing conflict, Grandiel locked himself in his magic tower. There, he remained out of sight until one day he rediscovered an energy that he knew very well, a black aura. The aura helped Grandiel find Baldinar, who was still very much alive in the form of Ashtaroth, as well as the group of young heroes fighting him.
Kassias Grandiel would then go on to assist these young heroes, teleporting them to his magic tower and forcing to fight insanely powerful illusions of creatures they had fought in the past as an attempt to strengthen them against Ashtaroth. The Grand Chase was not pleased with this, as it was completely unexpected and random and that Kassias kept his plan a secret until the very end.
Later, while the Chase were engaged in the battle against Ashtaroth, Kassias Grandiel opened a Dimensional Portal that led to the Monster Train 301 as an attempt to save the soul of their recently deceased friend while holding off Ashtaroth the best he could.
After the Chase arrive in Kounat's Collapse, Grandiel appears and informs them of Ashtaroth's presence, but the conversation is interrupted by Drawl. Grandiel leaves to confront Ashtaroth while the Chase deal with the demon.
Later, after Ashtaroth resurrects the Highlander leader Graham and forces him to fight Grand Chase, Grandiel arrives and uses the Klara Libri's clone to defeat him, but regretfully informs Sieghart that Graham and the Highlanders cannot be saved.
After Grand Chase defeat Ashtaroth in his demon form, he is surprised to be defeated and notes that it could only have been accomplished with Grandiel's training in the Wizard's Labyrinth. Grandiel shows up and reveals to have been his friend long ago. The elf tries to convince his old friend to stop, but Ashtaroth ignores him and uses the incomplete Aernas Hammer to transform into a god. Grandiel warns the Chase that their attacks are useless against Ashtaroth and uses the Klara Libri's clone to harm and defeat him while creating a barrier that nulls all of his attacks.
Upon God Ashtaroth's defeat, the Aernas Hammer suddenly warps its key, Mari, to the field, shocking everyone. Void immediately shows up afterward and denies Ashtaroth the key. Kassias Grandiel then takes advantage of this moment by sacrificing himself to seal himself, Ashtaroth, and the Aernas Hammer into the Wizard's Labyrinth where they will be trapped for all eternity, but not before wishing the Chase farewell.
In Grand Chase for kakao, Arme, to her surprise, met Grandiel after being saved by his students, Kyle and Cindy. While managing to survive, Grandiel became fragile enough that using magic could lead to dire consequences within his body.
Later on, the Asmodians that Grand Chase fought in the dimensional rift found their sanctuary and burned the place. Kyle attempted to save them but his soul took extreme damage from Belile's attack. To prevent it from perishing, Grandiel leads the group on a journey to reach the source of Prana in the world of Cricketia.
As a boss, Grandiel excels in magical attacks centralizing around lightning and illusions. His basic attacks include conjuring streams of lightning down on his foes and releasing a large, slow-moving lightning ball that homes in on his targets. He is also capable of summoning a circular array of lightning orbs that spread out from their point of origin.
When fighting Kassias, he also summons an illusion of Drawl and Veron, and four clones of himself after they have fallen, to assist him in battle. He cannot be damaged while Drawl and Veron are alive, and his clones can use all the attacks he can, though they obviously have less HP than the original.
- Kassias Grandiel is a very powerful mage, as seen when he was able to teleport the entire Grand Chase party (consisting of 13 powerful characters at the time, all deity-rivaling warriors, three being exceptionally close to deities (Amy, Jin, and Sieghart), and five of them being demons (Dio, Zero, Ley, Rufus, and Lass) into his labyrinth, create multiple waves of illusions that are much more powerful than the originals from their memories (Xenia deities included), set up Dimensional Portals, is the creator of the Phantom Phoenix Set, and is able to harm and defeat Ashtaroth in his god form. This could all be due to the Klara Libri's clone.
- Kassias Grandiel is one of two bosses with their full name known, the other being Talin Elfbane.