I totally understand how important the start is. I had to change my first chapters many times to finally have people read the second one XD.
-The cover/banner: The cover is good in itself, but the text is difficult to read. I would suggest making the title bigger and giving it some more pop. Maybe put 'The Last' and Elf in different lines with some stylized font?
If it's not too intrusive, here is a good reference for titling on a similar cover.
-How hooking the prologue is: Very much so. It has a lot of tension and builds intrigue. It's written very well. There are a few awkward spots and a few repetitive lines, but those things will not turn away any potential readers.
If anything, I would have ended the first chapter where Virion runs away from the battlefield with his mother since the atmosphere ahead is starkly different. But that is more of a personal opinion than a suggestion.
-Impressions on the first few chapters and whether they make me want to keep reading: I like them, the action is great, the tension sticks around and the plot seems very interesting. It kept me wanting to read more. I did find that there is a little too much information dumping in the first 5 chapters that could get a few people to turn away.
-General story and/or artwork: I said everything about this already 
-Style, tone, and setting: The tone is well set but there are a few instances when it shifts too fast. It can get hard to follow the story overtime. The styling is where a small, yet important problem lies. It's confusing to differentiate between his visions and the reality. I have just read the first 5 chapters and for now it seems like the visions are a important part of the story so this slight confusion can be problematic for a few people.
-Writing/storytelling style and how smooth it is: I really like the way you write. It sets the tone very well and the plot is exposed very quick and very nicely. It made me empathize with Virion easily.
As I mentioned earlier, there are a few awkward bits, but they are barely noticeable.
Overall, it is a great read and a really lovely addition to my subscription list. Very enjoyable, keep up the great work.