Here is another thing I noticed which ChatGPT has happily explained the distinction between:

“…which has arisen from a dark Imperial fortress…”
“Arisen” is the past participle form of “arise.” The verb “arise” generally means to begin, originate, or to come up. Using “arisen” emphasizes the emergence or origination of something new or something that wasn’t there before.

“…which has risen from a dark Imperial fortress…”
“Risen” is the past participle form of “rise.” The verb “rise” typically means to move from a lower position to a higher one. Using “risen” emphasizes the act of elevation or increasing in power or status.

Both can technically work in the context you provided, but they give slightly different shades of meaning:

If you use “arisen,” it emphasizes the FIRST ORDER originating or emerging as a new force from the fortress.
If you use “risen,” it emphasizes the FIRST ORDER ascending or increasing in power from the fortress.
Given the narrative nature of the passage and the notion of the FIRST ORDER’s growing power, “risen” might be slightly more fitting. However, “arisen” could also work if you want to emphasize the emergence of the FIRST ORDER as a new or distinct force. Choose based on the nuance you want to convey.

Me personally, I think “arisen” is the proper word here.