Tin Hanan/The Shadow (spoilers)

[Droopy]

Personally I interpreted the shadow to be Daniel's waning sanity incarnate. I figured that being in contact with the orb drives one insane and the visions are so real to the individual that they indeed cause them harm (the shadow growth hurts you, but in reality you hurt yourself. or something along those lines).

As other have said, the mystery and vagueness of the shadow is intended, but that said, it doesn't mean it can't be explored. The basis of leaving the slate blank is so the reader/watcher/listener/player will fill it in with their own fertile imagination, fresh from being stimulated with macabre imagery and suggestive information.

At the very least, this is an excellent opportunity for you to come up with your own answers to these questions, perhaps make your own book/movie/game/custom story with the concepts you come up with.

[higurekon]

I always thought of the Shadow as something that existed both as the cause and the effect of itself; if that sounds contradictory think of it this way: Daniel moved the orb from its place and therefore set into motion this universal force trying to balance time and space. Yet at the same time for some reason it grows stronger with every man that Daniel tortures and kills. It is as if it is related to Daniel's conscience, expanding as Daniel continues to commit murder and destroy his innocence. Eventually, after he tries to forget everything with the amnesia solution, the Shadow becomes something like his own realization of what he has done, chasing after him as he explores the castle and learns more and more of his dark recent past. It's very own existence has fulfilled its role as this kind of personal destroyer of Daniel.

[Fomzo]

I have the idea for a substantial Shadow's origin but this will be put into my fan'made sequel.

[Googolplex]

It called: Tin Hinan.

[Kein]

Alexander was the Shadow's target too, don't forget that. He not only had and extensively used an orb in the past, he also broke it. He'd been keeping the Shadow at bay for a long time already when he met Daniel. He was planning to make Daniel take the blame for both of them, it was his bad luck that in the revenge ending, it became him.

This is debatable, actually. As we know, Weyer was able to master the Orb(s) and take control not just over its powers but had no issues with the Guardians as well. Alexander as well was quite a strong person mentally (spirit and willpower), he used the orb already long before Daniel ever visited the Castle but Shadow didn't bother him before.