Warning: spoiler alert!Edit
Thiazzi is depicted as the "strongest man in the Forest" and ruthless. He was formerly the Oak Clan Mage, however, it is later revealed that the Oak Clan had died out. In the third book, Soul Eater, he is shown to take pleasure from harming the captured hunters, particularly Wolf - he is sadistic and arrogant. After Seshru's demise, Thiazzi single-handedly attempts to succeed in obtaining the fire-opal for selfish purposes (however, it may be argued that Eostra had commanded him to do it). Through creating rifts between the Deep Forest clans, the Oak Mage successfully creates apprehension and distrust between clans - this is his plan, as it weakens the stability and power of the Leaders as well as resolve. He did this by stealing children from the deep forest clans. The clans accused each other of and were able of killing anyone who didn't follow 'the path'. Thiazzi is one of the most powerful soul eaters, probably on part with Eostra. We can believe his mentioning that even the fire obeys him, judging to the fact he can stick his hand in the flames without beïng burnt.
It is Thiazzi who kills Bale by pushing him off a cliff at the start of Oath Breaker. This leads Torak to swear to avenge his kin, since he had abandoned Bale following an argument involving Renn. In doing this, Torak himself becomes ruthless, almost mirroring Thiazzi's ways, stopping at nothing to try and hunt the Oak Mage down. But Thiazzi knew exactly that Torak would follow him and left a bunch of deadly traps on the trail in the deep forest. While Torak ventures into the Deep Forest, Thiazzi stirs up trouble, and kills both the Forest Horse and Auroch Mages, and takes their places (by imitating them so that the clans do not sense that anything is different). This was only possible because the mask of the Forest Horse mage and the fact that the auroch almost always stays inside. Since he holds both these powerful positions, he convinces the Deep Forest clans, whom had previously been at war with each other, to unite and fight a war against the Open Forest clans - since they do not follow 'the Way'. 'The way' means the way the people live with the forest. The already existing rift between the open forest and the deep forest, created by the havoc of the demon bear and the sickness caused by Tenris.
It is also said that Thiazzi is Toraks kin because Toraks mothers father was oak clan so this means that two of Toraks kin had wanted to kill him now. The seal Mage and the oak Mage , Toraks had a rough few summers.