The Shadow question

[Hunter of Shadows]

I thought red goo is the residue left by the shadow, not his actual
form? Like some kind of marking to prove that "it was there"?

It is, it's just a manifestation of how it breaks down reality

The Shadow does not have any physical form, or any form that can be comprehended by a mortal human, remember that the Shadow is just the guardian of the Orb, and an effect of how moving the orb wasn't supposed to happen and the universe 'has to catch up with itself' or something like that.

Thus, the Shadow isn't really comprehensive, the red good, the red mist, are all manifestations of how alien and bizarre it is, in some ways it's like some of the Elder Gods from H.P. Lovecraft's Cthulu mythos, as the Shadow cannot be comprehended and has powers that warp reality just by existing