I have avoided Deus theories for a reason ^^. Posting some observations is fine, but the moment you get into the specifics they sound so alien. Time fuckery is cool but a fucking pain in the ass to conceptualize.
First things first, the script:
Deus vs MC said:
Deus "Arrived at last I see."
Deus "To think the it would split in a direction like this. Did I make a mistake? Or was it you... There's just too much for me to tell."
Deus "No matter. I can kill you, even from here, as long as I keep my eyes focused on this point. Then things will right themselves."
Deus "It's for you own good, believe me. I'll put you out of all our misery."
Deus "......"
Deus "Tch, this is taking too long. It's too hard to see you from here."
Deus "You've progressed too far; I need to find another point."
Breaking it down piece by piece:
A) Deus "Arrived at last I see."
This may seem like a greeting, but it also signifies Deus could pinpoint the exact location MC will turn up, even in such a small timeframe. It's unlikely that any significant time passed since MC was possessed, up to engaging Deus. While it doesn't seem an amazing piece of information, it shows that Deus can predict events on the fly and he doesn't have to "prepare".
B) Deus "To think the it would split in a direction like this. Did I make a mistake? Or was it you... There's just too much for me to tell."
The word "split" may be significant, but I'm not sure what to make out of it yet. However the next sentences paint a picture. Deus wonders if he made a mistake or if the MC caused this situation. These are fairly important:
Observation 1) Deus can be mistaken.
Explanation: If so, he doesn't just "see" events unfold. He has some control over these events. The question is, is the mistake related to his personal actions (for example, he forgot to say something important during a conversation with MC), or it's related to his power (for example, he has to actively dismiss branching directions because they are too numerous and too draining, so he chose the wrong branch?).
Observation 2) MC can affect the end result.
Explanation: This introduces a completely new interaction. I would like to call it "free will" or "unbound destiny", but I think it has to do more with power. My theory is that everything in the universe is created by the same powers that the Arbiters (and by association the Apostles) hold, so an exceedingly powerful individual can affect the "fundamental laws" of the universe. But without delving too much into my theory, we should take it as a very important fact that an individual may be able to alter a "predetermined" result or introduce something that Deus can't see.
C) Deus "No matter. I can kill you, even from here, as long as I keep my eyes focused on this point. Then things will right themselves."
This is where things start to get murky. The assumption here is that Deus' eyes can see "points". The nature of these points will probably give the answer to how his powers work but sadly the info we have is very limited. The second part of this quote however is the more "crazy" one. Things will "right themselves". This is basically saying that Deus isn't the one who is fighting MC. He may or may not be the source of that beam, but I think that this isn't the actual fight and it all plays out on a very different scale.
D) Deus "It's for you own good, believe me. I'll put you out of all our misery."
A small thing here. Deus likes the word "miserable". The interesting thing in this sentence is the "all our misery". I don't know what to make out of it so I'll just leave it here for others to note.
E) Deus "Tch, this is taking too long. It's too hard to see you from here."
Both of these sentences are juicy
Observation 1) Deus saying "this is taking too long" is probably referring to C. To put it all together: "It is taking too long for things to right themselves". He also gives us another piece of information, that it's too hard to see MC from whatever "here" is.
F) Deus "You've progressed too far; I need to find another point."
This is also an odd thing to say because it's hard to understand what "progressed" means in his context. The easy explanation is that he is referring to MC becoming way too strong, but I said earlier, the true fight is taking place somewhere else. Another way to read it is "you've progressed too far for things to right themselves, I need to find another point where if I focus on, things will right themselves faster".
Observation 1) The "too far" would refer to the "physical" location of the point in time. If the branching point is MC's evolution, perhaps the "weight" of this event is so significant that even mere minutes make it impossible for the last point to have influence on whatever is currently happening.
Bringing them all together:
The way I see it, Deus power is "branching point manipulation". The nature of these points are a mystery. My assumption is that they are part of a cosmic order and not something that he creates, although he may have the capacity to create new ones for his own personal use.
A being that exists on a branching point needs to make a decision. This decision will close off the other branches and the being will continue existing on the one it chose.
Deus has the capacity to move in time and access the branching points he needs. By focusing on them he creates an effect that makes reality "fix itself". My theory is that Deus can pull the being back to a branching point and force it to decide again and again and again till it makes a decision that Deus wants. Once that happens, the being will "change direction" and reality will fix itself like the branching never happened.
A very powerful being like a god may have the capacity to break those branching rules. Perhaps if possessed MC was complete, he would have progressed "too far" in a very short time for Deus to pull him back to the branching point and losing his grip on this reality. it "makes sense" that the further someone moves from the branch, the harder it becomes for Deus to pull it back.
He claims that he can corner the Hexenringe if it feels threatened. A threatened Hexenringe would most likely flee so Deus can pull it back again and again till it makes a different decision. Perhaps Deus can travel further back in time to change another branch that will increase the likelyhood that Hexenringe will act like Deus wants it to.
I do have a guess on how Deus's power works so hear me out gents:
If the timeline stuff is all correct that means Deus exists at various points in time and time has branched off into various branches which Deus can see only because HE exists at those points.
My main objection with Deus "existing" in the branching points is that in his battle with MC, he physically left the battlespace and moved through the rift he created, ending somewhere else in both space and time. If he could simultationally exist in all these points, then he wouldn't need to leave.
I can imagine that Deus can't see super far into time because something happens (aka he dies) and I'm going to guess that Deus cannot see into a certain point in the past because he doesn't have his power at that point in time.
I think Deus power is a cosmic force. It doesn't matter if he was alive or dead at some point in time, he can travel to it. The range is probably determined by how powerful he is. My assumption is that Deus can travel to the past easier than the future, but affecting the past gets progressively harder the further he moves from his current existence. To travel to the future he has to identify and travel through all possible branches, which takes a lot of time. If I had to guess, I'd say that predicting big events that will happen in a week, is easier than predicting small events that may happen tommorow, because to reach big events he can travel through easily identifiable branches, while the smaller events are numerous and insignificant.
At some point Deus claims that "there are just too much for him to tell". This could actually be a bit of fourth wall breaking but it seems that some branches have so many possible directions that Deus has to pick which of them he is going to follow. The fourth wall breaking could be the deadend choices during MC vs Deryl.
So yeah, essentially Deus is a shepherd. Everytime a sheep (target) leaves the path (what Deus considers the main branch), Deus has the power to pull it back enough times for the sheep to make another decision. He focuses on the branching point, sheep is pulled back, reality closes off the alternate path and everyone is happy.
Edit: Some proof reading, rewriting unreadable parts and fixing spelling mistakes.
