Zathroth is definitely more important than Ferumbras- as he is a god. Or perhaps we should say the evil half of the god... genesis explains this as following:
“ n this emptiness two powerful entities appeared who were eventually known as the elder gods: Fardos the Creator and Uman Zathroth who combined in himself two unequal halves. One of these halves was Uman the Wise, a benign god who was gifted with divine intellect, while Zathroth the Destroyer was the other, darker half. These were the two halves of one single enigmatical entity, and even though either of them was perfectly capable of acting on his own just as if he had been fully independent, independent they were not. They were bound together by an eternal bond that could not be broken, and their destiny was one.‘
Zathroth is a god of corruption, and cunning. He has agreed to help other gods in creation of the time. He add “seed of destruction” to it, which caused “time” being with the flaw- “ He understood that a world that was subject to the ceaseless passing of time would be doomed to perish slowly”.
Zathroth filled with jealousy no hater murdered Tibiasula.
Uman and Fardos in desperation of trying to save Tibiasula bind her body to column of time- and this is how the first creation been done - the elements : Tibia- Earth, Sula- Water, Air coating both of them and fire to warm the earth. And later from those new gods been created. Then Zathroth seduced one of the new gods- Fafnar’s, which give the birth to the Ragin Titan - Brog- who later created cyclops, first dragon Garsharak, trolls, goblins. As they become his minions- they caused destruction and chaos for years- and this was named Age of Chaos.
He later sired another god- Ulgith- Master of Undead. “ Urgith was a cruel god who strove to infuse the bodies of the dead with unholy energy, dooming them to a state that was neither life nor death. Thus, the hour of Urgith's birth marked the beginning of undeath.”
Zathroth also managed to corrupt some of the Ancients and it is rumoured that they were formed into first Demons. It is also to be claimed thatZathroth created first Demonic Lords from raw form of magic powers which was designed for Banor’s Twin brother: Yes, some even state that the good Gods created a twin for Banor, which should embody the powers of magic, whose raw form however was stolen by Zathroth, and he had manufactured from it the first daemonic lords to lead his attendants from hell.
Zathroth is one of the dark gods who managed to create the first Vampire (Dark Woman) which later the reason of Banor’s fall from the grace of the good gods changing him into Vampire.
Now, Ferumbras is the follower of Zathroth. He somehow got the access to Zathroth’s library where he found the knowledge and key to his power. “ Others attribute this to his bloodline, which included the likes of Goshnar and dates back to the first magician kings of Ankrahmun who unified their people against the lizard threat‘’
https://www.tibiaqa.com/823/what-happened-to-banor?show=823#q823
https://www.tibiaqa.com/24809/who-was-the-first-vampire-and-what-was-the-beginning-the-vampirism-tibia?show=24809#q24809
https://www.tibia.com/library/?subtopic=genesis&page=1
https://www.tibia.com/library/?subtopic=genesis&page=2
https://www.tibia.com/library/?subtopic=genesis&page=3
https://www.tibia.com/library/?subtopic=genesis&page=4
https://www.tibia.com/library/?subtopic=genesis&page=5
https://tibia.fandom.com/wiki/The_Ferumbras_Files_(Book)
https://tibia.fandom.com/wiki/The_Creation_of_Humans_I_(Book)