This is going to take some explaining, but the basic idea is thus:
In the world I'm creating, if matter is exposed to enough magical energy it ceases to respect the laws of physics and becomes known as "eldritch matter."
Eldritch matter vaguely retains it's original shape and but on a microscopic level it's essentially a solid with the properties of a liquid (and floats) that retains a shape at a general level, but no complex structures are able to be maintained as the matter constantly shifts.
There are some animals that have become eldritch matter, and when they do they survive, and become and eldritch entity, a creature that defies the laws of physics but in return has no neurological function because their neurons are not able to be maintained, so they are essentially reduced to the lowest form of intelligence possible, a mindless beast that does nothing but endlessly seek magical sources to suck dry to maintain it's existence.
Since they are essentially immortal so long as more magical sources can be found, people have tried multiple times to become eldritch matter, the basic idea being maintain the structures of the brain while all else becomes eldritch matter. This almost never works, as eldritch matter tends to spread, but one person has achieved it.
They now take the form of an amorphous mass that has been crafted to resemble their previous form, but their brain structure has been maintained so they can retain control over it. However, over time their neurons very slowly turned into eldritch matter, destroying the structure and sending them essentially mad.
This is (very roughly) equivalent to a neurodegenerative disease, as parts of the brain slowly become useless, but it would be attacking every part of the brain at once, albeit extremely slowly and from the outside. I was wondering what irl disease is the closest equivalent so I can model their behaviour on that of sufferers of that disease?
p.s. i have no basis to know if any of this is correct or plausible