It's important to set the scene, especially in an online format like this where someone may start reading your newest piece without having read would you wrote weeks or months earlier. Even though I read the earlier pieces, it took a long while to re-establish some of the scene details in my head -- even the very basic fact that they were underwater. It also would have been good to explain more of what was sealed, who and what Kobvan was/is, before everything went to hell.
I know some of the details from the back and forth of your submissions, but a lot of those details were avoided or glossed over in the chapters. I'm not even completely sure what Kobvan looks like. Demons can look like many things, after all. I had thought of the traditional winged demon at first, but there was mention of a quadruped, also.
The battle started out good, but I think you lost your stride when Velden was knocked out. There's a passing note that she wakes up hours later (I'm not sure if this is meant to be literal or just feel like hours later), and you have Gwen fighting with a sword, assumed to be from a Sword Rain. Fair enough, but then you swap perspectives to Gwen, showing her losing momentum until surprise, Velden saves her. If you planned to write from Gwen's perspective, it would have been better to skip the part of Velden waking up, show a little more of Gwen fighting, and then it raises the suspense until Velden comes in with the save. Even here, the fight's not bad, yet.
It's after that, when Gwen is dying, the demon is dying, and Velden is dying, that the sense of urgency is just absent. Velden leaves, hunts for half an hour for a gem, then returns, casts several spells and rituals, at least one of which you describe as feeling like it takes hours. The best comparison I can draw for you would be FMA, when Ed has to save Al's soul after the human transmutation. He's bleeding out from the leg, realizes his brother is gone, and in a snap judgment and moment of panic, he uses his newfound knowledge to imbue his brother's soul into the nearest thing at hand, the suit of armor. Even rushing, he nearly dies from the bloodloss and Al has to rush his body over to Pinako.
That urgency was needed here, a sense that Gwen's life was in danger, that Velden is making a sacrifice for her. The way this is written, it feels like the only thing sacrificed was time and pulveris, when I know that wasn't the intent.
Mechanically, dialogue punctuation is always something to pay attention to, particularly when transitioning out of an exclamation or a question. Internal monologue is usually denoted in italics, and in a few spots you do this, but in most you don't. The horizontal lines were placed in odd locations. I couldn't tell why some of them were there. The spelling and verbiage, however, was good and your sentence and paragraph flow was also pretty good. Your biggest issues were just the punctuation and the coding.