After first turning Wyndham into his demonic capital base, and aiming to take down the Holy See stronghold of Vritanis to the west, Ganishka would agree to a draw with Griffith for a final battle in Wyndham, after Griffith with his New Band of the Hawk had halted Ganishka's attempt at taking down Vritanis.
Ganishka then returned to his capital to preform a ritual involving absorbing the strength and lives of everyone nearby in order to "Transcend the Apostles". He believed this was necessary to beat Griffith, and the process was described as "using an artificial behelit", meaning it could be used as a means to reincarnate into being superior to what it formerly was. His new form is a massive monster as high as the sky, with hundreds of tentacles whose destructive power was compared to that of "shiva".
The circumstances of Ganishka's transformation into an Apostle are shown in chapter 303, through a flashback into his childhood, where it's revealed that his mother tried to poison him to ensure the throne to his younger brother, who later was killed, and his mother committed suicide. The king, later, lost his life in a trip, when he fell off his mount after being hit by a toxic dart. The throne then passed on to Ganishka, who took the decision that only fear could ensure his survival, but it prevented him of his duties as a father and husband. Years of conquest later, when he was in a feast for one of his victorious campaigns, his own son tried to kill him. Moments from death, his bloodied hand grasped the behelit, which was revealed to have been given to him by Dhaiva. His son was sacrificed and Ganishka was reborn as an apostle.
It is seen that Ganishka, The Snake Prince, never really was at peace. He was at a constant state of fear and mistrust. He spread these feelings around through his conquests. The son who killed him shared the same feelings as his father and looked his younger self.
Shiva is the name given to Ganishka by his followers during his transformation. He used the same "rig" of apostles that he used to create his demon soldiers. When he lowered himself, he disappeared in the liquid. A slow creeping fog killed everyone in the area, except for Dhaiva because he erected a personal barrier to protect himself. This was noted to be a "Man-Made Behelit."
In this state Ganishka looks like a two-legged tower, from far away. His head is a mound of multiple heads and they can all breath fire. There are many whip-like arms. Ganishka can also create apostle-spawns of himself. All uniformly looking like big heads with sharper teeth and tentacles where the neck should be in order to move around. Also, probably because of the power that he attained by his own way, he lost all sense of rational thinking and self (much like Guts Berserker rage):The emperor seems to be ignorant of the effect of transformation on his body. He is unable to register the fact that he is now a huge entity and thinks of everyone around him as insects.It can be observed that his mental state deteriorates as he stays in this form. At first he is able to recognition Dhaiva . But then he is disorientated and starts to get lost in his own power .He loses his sense of perception.He crushes his Army under his footsteps and considers the gore thus produced as "Crimson Flowers". He is capable of emitting fire from his many mouths. Dhaiva notices that his ability to scorch the earth by mere whim is similar to that of Shiva (the God of Destruction as per Hindu Mythology).
Ganishka was confirmed as being dead, apparently after receiving a blow from Skull Knight's Sword of Resonance that was deflected by Femto by warping the space around him. This triggered the "World Transformation", which caused his corpse to form a giant tree (described to be much like holy trees from many religious stories) from the base of which sprouted the mysterious new castle "Falconia," implied to be Griffith's new kingdom.
Ganishka is said to have become the light which he seeked. The light engulfed the entire world merging reality and fantasy, the real world and Astral world. This causes the appearance of Dragons, Trolls, Goblins, Hydra, Fairies, Unicorns and other such mystical creatures on Earth.