One of the main features of update 8.4 was the integration of the city of Yalahar, where we also find the Infernalists for the first time. According to the game's story, Yalahar was founded by a group of scientists and technology experts who wanted to create a futuristic and advanced city. However, the city was also in the sights of the powerful mage Variphor, who tried to take control of the city and its inhabitants.
Variphor was a very powerful and evil mage who wanted to gain control of Yalahar and its advanced technological resources for his own dark purposes. To achieve his goal, he began to influence the city and its inhabitants, corrupting many of them. Variphor started to manipulate the city's technology for his own purposes, causing damage and destruction in the city and its inhabitants.
On the other hand, in Ferumbras' Ascension Quest, Mazarius, who has the parts of The Godbreaker, reveals that he is working for Variphor through a letter that suggests that Mazarius was working with Variphor and manipulating the players in their attempt to stop Ferumbras' ascension to divinity. Mazarius indicates that Variphor's influence was behind the players' actions and that their efforts were part of Variphor's plan to use the Godbreaker and open a breach for his arrival. Mazarius also mentions that Ferumbras was an ideal candidate for his plan because of his vanity and lust for power. In the letter, Mazarius mentions that he will use the coordinates provided by Variphor to open a portal to Ferumbras' hideout, and the players will have to confront him during his ascent. The letter also suggests that regardless of the success or failure of the players, the energies collected during Ferumbras' ascent will be used to strengthen the breach and allow Variphor to enter fully.
It is possible that the Infernalists obtained Ferumbras' hat after his defeat and decided to use it as a symbol of their power and loyalty to Variphor. I should note that the information provided in the game does not indicate that the Infernalists work directly for Variphor. However, given their demonic nature and presence in Yalahar, it is possible that they are influenced by him or his followers.
Another simpler answer would be that the Infernalists are magicians and what they are wearing is not a Ferumbra's hat, but another hat used by magicians such as a mage hat, magician hat, or even the old sprite of the hat of the mad.
Sources:
https://tibia.fandom.com/wiki/Updates/8.40
https://tibia.fandom.com/wiki/Yalahar
https://tibia.fandom.com/wiki/Variphor
https://tibia.fandom.com/wiki/Ferumbras%27_Ascension_Quest#Ferumbras
https://tibia.fandom.com/wiki/Unfinished_Letter_of_Mazarius_(Book)
https://tibia.fandom.com/wiki/Magician_Hat
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