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My personal hc about the Sanctums - both this and Stone one, is that they used to be hospitals. I also think snails' mounds were also some kind of clinics but not nearly as large. With Soul Sanctum, I think it was also some kind of medical university, mostly regarding the uses of the soul (hence the name), which requires to it to be not only a healing item but also a weapon to help fighting off harmful microorganisms.
The Soul Master. Well, firstly, I hc him to be a brother to Sprint Master and Dash Master. During his youth he's been called Jump Master. I also hc No-Eyes to be their sister, called Float Master. Don't think those were their actual names, though. I hc that they were all children of the Stone Sanctum leader, who tended to those coming through the Pilgrim Way - a pretty dangerous place for most Hallownest bugs. No-Eyes inherited it because she was the eldest and most skilled - mentally and physically.
The future Soul Master went to the capital, hoping to achieve even greater things at a Soul Sanctum. Which he did. He quickly became the star student, capturing the attention of the current leader, who later made him his right hand. Since that leader didn't have any heirs - non of their children ever showed interest in medicine, it wasn't too hard for SM to take the place of a leader after their retirement.
I personally hc SM to be not too bad of a person before the Infection. I imagine him to be very ambitious, slightly pomptious and arrogant, but also sincerelly caring towards the patients, of both - higher and lower birth. I hc him, Monomon and Lurien to be in a pretty good relationship with one another.
When the infection had stricken he was in a panic. He even begged the King to help him with researches by supplying the Sanctum with more soul, since the King is a pale being. However the Wyrm forsaw the massacre SM will commit without seeing the reasons and asumed this to happen as a result of him oblidging. Not only he denied the SM's request but strictly forbade him to attempt any research regarding the cure and just perform palliative care to all of the infected. That cause SM to start resenting the King. He asumed the only reason of his refusal was the arrogance and jealousy towards the mortal. This resentment only grew worse when his sister has met her tragic demise because of the infection (points at No-Eyes' backstory). The Lurien, as usual, fully supported the King, which really soured the relationship between him and SM. Also I hc that after the whole vessel plan was fullfilled, despite being in peace about the Infection (not for long as we know) SM felt a little insulted that the King didn't even thought to offer him to becomes a Dreamer, but making a Beast of Deepnest to be one (classical bug racism).
I imagine SM becoming the selfish, hubristic, mad scientist we see him as, during the second wave of the infection. The same wave that caused the King to hide and the Kingdom to fall. No longer does he care about the well-being of his patients, especially those of a lower birth. He only uses them as tools to prolong his and his coleagues' existence. And no longer does he care about the infection. He only considers it as another tool, while we know it was entirely other way around.
Soul Sanctum. What are y'all's thoughts, impressions, and/or headcanons about the place and the bugs within?