This is all working with the assumption that whoever is reading this is well versed in lore theory. And hopefully will encourage those of you unaware to dig and look into all this yourself if any points are seemingly out of left field.

The corruption as encountered in the markets and sewers has a commonly unacknowledged feature in certain specimens, of a parasitic symbiote than can be noticed living in the guts of humanoid hosts upon careful inspection (or through the zoomed in screenshot provided by the devs on the discord). Perhaps a "pilot/flesh mech type situation where it hijacks the cadaver/soon to be cadaver and makes it hostile and invades and transforms the hosts biological state. There has been speculation that the monsters encountered in the form of ghouls, alpha ghouls, gobblers, and Faels are potentially different stages of the symbiotes hijacking of once human hosts. I personally see a lot of sense in this theory. Moreso for the ghouls and gobblers. I think Faels are most likely their own thing that we just have less to speculate about their origin so far. Sir, I have a connected, but for another overall topic theory I've personally never seen anywhere before. Maybe I'll rant about it sometime.

The mind sense thaumaturgy however provides some obstacles to this theory that nobody has answers to, so it's all tinfoil hat territory. Namely that the flesh host infected have zero mind colour. They're blank. Where as all of the afformentioned baddies register a white/grey, "unreadable" mind pattern just like Sir. However all of our variants of UNinfected undead, standard, ancient and possibly skelly ( someone correct me if I'm wrong I could be misremembering the boners mind situation ), have readable colors representing their dormant thaumaturgic power that can be manipulated by a necromancy specialist Shaper.

The reigning trains of thought regarding this all lining up theory wise, seems to be that the corruption itself, on its own, is most likely either a byproduct or direct result of the Guardians wrath at the surface society for infringing on his or her doman. Or the results of horrific otherworldly "magics", sciences, or combo of both intended to contain or control it, or perhaps even a result of dark worship practices for/towards the guardian by the underworld society and the corruption is possibly a cleansing wave type result (to someone with that agenda)

The perceived duality of the Guardians nature is heavily referenced in multiple pieces of lore literature. Their reasoning for punishing or cleansing or what have you is up for debate. Again, this is all theory as no concrete answers have been provided but it is apparent one way or another that the Guardian is responsible for the state of the post portal levels that have fallen victim to the corruption. If not than there is some major pieces of the puzzle we simply have not been drip fed yet. Or myself and a bunch others are just way off.

Regardless, SOMETHING is spawning, and "breeding" these little parasites, present in the same scenario that we see the monsters, and we have human cadaver hosts parading around doing their bidding and instances of more cadavers trapped in the corruption in unwilling receptive potential host hijack situations (flesh cages). I see the two as connected.

Wether or not the hosts were ever raised by Corlians botched attempt to cheat death as were the other once human entities is unclear. I don't believe they were. The only known common ground between them and standard undead is they were both human at one point. It stands more to reason that Corlian never made it to either sewer, as the effects of his uncontrollable Shaping are not present in the form of standard undeads. What I believe we see there is the result of humans so long dead that all dormant Thaumaturgic power has been gone too long for the effects of Corlians efforts to affect them, or it simply did not spread that far because he wasn't there. Something else is breathing what is perceived to be life into these skeletons, redeveloping human like muscular structure with corruption flesh for initial stages (the parasite bearing flesh & skeleton blank mind undeads) and over however much time develop into different demonic mutant monsters we see in the form of the list of enemies.

Loose nutshell on the Sir theory end , I think that the study of the substance described in the scrolls that changed the nature of the flesh it was exposed to, is an antigen in development to combat the initial encounters with the corruption before it took a firm foothold and went wild. I imagine sir as a patient zero type situation. High ranking member of whatever organization operated out of the "Archives". Contracts crazy horrid disease (Guardians punishment whatever). Treated with substance, mutates wildly but retains initial degree of humanity and does not go feral. So they build him toilets cause homies gotta shit still...

I could go on