This might never get to be cannon, also, this is not only about the mask itself. It is about the mask and what it shows.
It has been discussed on the podcast about using eldritch horror themes as background story for the lore. Annona being the closes thing to one such beings as well as this post being another. I say close to one such beings because eldritch horror is characterized by the impotence of the main characters to do anything.
Moving on to the mask of insight, this mask was not created by any of the three brothers, nor by their father. It was created along their father, as an aspect of the universe itself, similar to the infinity stones. Whether it was created by someone or has an origin similar to the infinity stones is not relevant. It is its power what grabs the mind and slowly flays it.
The mask as it name states, is the mask of insight, allowing the wearer to acquire a deep understanding of the things they see. Insight starts slowly, beings being ignorant of the things they live in, and in moderation it proves powerful. Allowing the wearer to understand people, physics and even powers. However, the mask never stops, slowly creeping into the creation of life, to creation of the great beings, to the unspeakable abominations that plagued the chaos of creation. Many care little for the tiny little kids and their struggles. Ignoring Makuta, Ekimu and Karzahni as their existence is alien that they might not even recognize the three as actual beings. However, to gaze upon the abyss is to allow the abyss to gaze back at you, and the mask of insight are the eyes that see. Coming into contact with such beings results in a primal fear, not of the things themselves, but of the unknown they represent. Prolonged use of the mask often results in the wearer slowly losing their grip on the present, followed by a loss on their grip on reality. Insanity is unimpressive, suicide being the most common.
Learn of your place in the universe and how little the universe cares about your insignificance
In the entire cannon story, only Karzahni has come in contact with the mask of insight. Although brief, it allowed him to understand the value of knowledge, and help him forge his mask of alternate futures. The newly forged mask allowed him to prevent his descent into madness, by showing him the consequences of continuous use of the mask of insight. Ironically, his mask of alternate futures would eventually be the cause of his descent into a different kind of madness.