Griffith's dream was to become a ruler of a kingdom.
While it has always been true for any form of Griffith (purebred human, angel Femto, reincarnation into human) that his goal is to have a kingdom, he has never mentioned the boundaries of or the terms for his kingdom. That is, a reader in early chapters of the manga could think of it as the kingdom of Midland; in middle chapters, as Empire of Holy See and of Ganishka; in recent chapters, a kingdom encompassing both Human and Astral World.
But at what point did his dream all go to hell?
As it is already clear in the manga, this dream of having his kingdom is the reason why Griffith in his human form lived, later took rebirth into an Angel (to get rid of human body restraints) to start his expedition of having a kingdom again (because of being feeling lost in the jail for so long when he was purebred human), and had reincarnation into a human to claim human world from the existing emperors. Accounting the development in recent chapters of the manga, there is no doubt that this dream has not been abandoned but has been invigorated with new plans.
In your words,
the dream did not got to hell but the hell has been brought to earth by him to rule both simultaneously from Windham.
How did demons originally get involved in all this...?
Some part of the following text is complete opinion of mine.
Humans follow the fate set forth by The Idea of Evil and the Behelit which the chosen ones receive draw them to the currents of causality, thus, reaching a point of despair and malice, that they decide to offer sacrifice to get rid of this feeling.
Turning into an Apostle doesn't end fate but marks a watershed moment. They are still headed for the destiny set forth by fate knowingly (like Emperor Ganishka thought) or unknowingly (like the tragedy with Count).
As for Griffith, his fate was set by The Idea of Evil and the Crimson Behelit made sure that he remains drawn to the currents of causality as answered by Gunfulker.
Now, to the question about involvement of the Demons.
Regarding their involvement into the eclipse
The Apostles might knew or could have received the information that their will be a feast on eclipse, or an angel, a member of God Hand, will be reborn, or it could be both. So, they came to the spot en masse.
The aforesaid conclusion can be deduced from the following events in the manga:
- When Wyald tried to look for Crimson Behelit in Griffith's body (to call God Hand) during latter's rescue by his band;
- When Zodd interfered and killed Wyald for doing such and later accepted the position of front guard during feast;
- When Rosine and Count among others ate injured members of the band of the Hawk (while Rickert was away to fetch water) in the night; and Skull Knight asked them to make haste for reaching the Eclipse spot.
Regarding the involvement of Demons in the army of War-Demons
It was not shown in the manga but can be concluded that, since Zodd was seeking the strongest one to fight (as he witnessed the dream of a white hawk flying in a dark sky) he was defeated by Femto, thereby, Zodd became the first member of Griffith's army. It cannot be said how Locus, Grunbeld and Rakshas came to know of Griffith's presence on Earth, but they chose to join him voluntarily as witnessed in the manga.
It has also been witnessed that a demon cannot stand against Femto, irrespective of their wish. When Ganishka (extremely powerful Emperor and a demon) encountered Griffith the first time on the battlefield he felt been carried away by latter's presence, comforting voice, and the blood of demon realm that flows through Griffith too. Therefore, it can be concluded that while an Angel doesn't control a demon willingly, a demon cannot directly fight against, as well as oppose an Angel.
Hence, it can be ascertained now how did demons get involve with Griffith into his War-Demon army.
Note: The question has been answered by accounting episodes 01-337.