You've killed us both

[thePyro13]

I don't think The Shadow is a guardian. After all, Weyer managed to remove several orbs from their resting places without invoking the shadow.

I think it is a side effect of someone who is not skilled with the orb trying to use its power. It chased Agrippa when he first activated one, and daniel, but not Alexander, or Weyer.

If the shadow is somehow related to daniels latent connection to the orb he activated, and most of the flashbacks seem to be about Daniels past(the one about the hitting a child with a rock in particular). Then I think the shadow chases daniel because he feels guilty for the things he has done in the past. I also think the shadow become stronger based on the strength of Daniels guilt. When alexander tricks him into thinking he warded the shadow off, the shadow backed off(he felt as though he had paid away his guilt). And when he realised what was going on, and hated himself for what alexander had made him do and the shadow came back in full force.

Likewise, the shadow backed off when daniel wiped his memory, and picked up pace as he found out more about what he had been doing with alexander.

Now the endings:

Ending 1: Alexander escapes leaving Daniel behind. Daniel feels beaten and wasted, without anyway to atone for what he has done. The shadow consumes him.

Ending 2: Daniel lets Agrippa through the portal. He still hasn't faced what he has done, the shadow consumes him, and Alexander. Though in the end we hear Agrippa plead to Weyer to save Daniel, so we don't know for sure of daniels fate.

Ending 3: Daniel places the blame for everything he has done on Alexander, and ruins Alexander's plans. He wants Alexander to pay for what he has made Daniel do. He breaks the portal, and the shadow consumes Alexander. Daniel feels as though he has received justice for what happened at the castle and the shadow retreats.

This is the theory my brother and I came up with. Though I'd have to read through many of the diary's(particularly Agrippas letter) more carefully to say for sure and to find any mistakes.