The Zodiarchs were originally known as The Council of Twelve, a group of mages appointed by the Wizard King to help govern his realm. Over time, they became corrupted and plotted together to kill the Wizard King in order to gain total control and to use Cassiopeia as a puppet ruler.
Horace, the sage of ages, who was in charge of teaching Cassiopeia magic, created the Wizard's companion to aid in her magical education. However, before he got the chance to teach her the wrongs of using the forbidden spell, Manna, he fled in fear that the Zodiarchs were plotting to kill him as well. As a ghost, he regrets fleeing and wished that he had stayed by Cassiopeia's side.
They died in the first use of the forbidden spell, but were recreated in illusory form by the White Witch. Although it is not entirely explained how, it is later revealed that not all of the Zodiarchs were evil, since Gallus was shown to be of great assistance to Oliver and particularly Pea. However, most of the phantom Council was still maliciously aligned and together they merge into one being to serve as the game's final boss. It can be argued that the council are the main antagonists behind Cassiopeia's descent into despair, as they are the cause of all her suffering.
Not much is known about the members of the Zodiarch except that Gallus was the 10th member. He was later revealed to be the ghost spirit of the wand Astra, and that Cassiopeia was his daughter, implying that he is the ghost of the Wizard King.
The helmets of the Zodiarchs each resemble an animal of the Zodiarch, and each of their staffs are slightly different, depending on the Zodiarch animal they represent.