5. The comma seems like a momentary verbal pause to me, rather than a grammatical one. I say keep it.

6. sounds like a tense shift to me, even though it probably isn’t, in either form. The way it’s phrased in the game, you have “After” (expressing potentiality)…”can seal” (potentiality again)…”happens” (present). Perhaps “so such a catastrophe shall never happen/come to be/come to pass again”.

7. consider “to the benefit of the continent”

8. “but” == “however” here, so you it’s not needed, but it can be useful

11. I would actually use “[the] awesome Langrisser” here as an antonym to “dreaded”…but the vast majority of players would not be familiar with its “classic” (read: now archaic) meaning, “worthy of awe” — so it would just be confusing. it does kinda need a “the”, though.

13. reasons comma

14. it means “held sway [over]” in this case. as in “reinforced [her] command”

Within the castle walls,
the strength of both sides'
missions, emotions and
ideals will be tested.

2. It could be a dangling modifier, but I kind of like the sound of the current sentence.
9. Will do lives, but I would prefer to keep the but. I see it as a more natural way of countering his “I can’t promise anything.” Not necessary, but it sounds like what you would say, and Rohga is … less cordial.
12. Yes … but you don’t know that until another game
14. How about:

Her beautiful, showy appearance
and her cold, merciless nature
helped Imelda maintain
strict control over her division.