I think the skin is beautiful and the detail in it is great, but it wasn't what I was expecting. As a big Nocturne lover and a fan of playing him, I have all kinds of ideas on how the skin could be improved.
As seen on the teaser for Eternum Nocturne, it says "Beyond Space and Time" in the title as well as that he "tears through space and time" yet so far in the skin there really isn't much related to space and time, at least you wouldn't think there was on first glance unless you saw the name. I've said it before and I will say it again, this skin needs more "wibbly wobbly timey wimey stuff".
So let's start with looks. While we are used to Nocturne not really having a tail since he is a ghost, this skin is legendary since it is supposed to completely change his style more or less as many legendary skins do. So having a physical tail is one of them. I'm not completely against it, but it is too large compared to his body. I would thin it out a bit and maybe shorten it just a little. Should be sleeker. Just makes more sense since he is supposed to be an assassin. Either that or make his shoulder pads larger and more decorated. They look too small and plain right now. Let's get some spike action going.
A big complaint among the community in regards to the skin is that he looks much more robotic than a being of time and space. Just put some more time and space relation into the appearance somehow and I am happy. Right now on the PBE, there were some particle updates to his ultimate that distorted the space around him during the duration of it though towards the end the particles are not easily noticed and probably wouldn't be during intense game play/team fights, though adding them passively to the skin would give it a little extra constant time and space "umph".
The current state of Duskbringer (Q) is pretty awesome, I have no problems with it really except for the trail though I don't know what I would do to change it. Moving on.
Shroud of Darkness (W) is a spell shield, but it doesn't look like anything special. It would be cool if it was a spell shield that was form fitting to his figure. Wrap around him you know? Or change his body as we see in the beginning of his taunt. Would just be a way to make it different. Perhaps when it absorbs an ability, give him an extra effect with the AS boost, distortion around him maybe? Yes, I like distortion. It is in the ult, but I don't think it is noticeable enough after the "jump".
Unspeakable Horror (E) isn't bad in it's current state either. It isn't as flashy as the cast for Duskbringer, but still pretty awesome.
Paranoia (R) was disappointing when the skin was first shown on the PBE and still slightly is with the changes added to it. Pretty much, he casts it, the screen goes black for everyone just like it used to, except a noticeable black hole-like particle appears swirling around him, then he dashes to the target while spinning, hits them, and keeps a barely noticeable distortion around him. I have a few ideas of how this ability could be made legendary. First idea, continue with the distortion, give the current after distortion to him passively, its a small amount, make it much larger and noticeable with his ultimate active though as it is when he casts it. Second, distortion around the whole screen. We've seen Lux's Lucent Singularity (her AoE slow) and how it distorts the area around it, almost sucking it into its center point, as well as the screen distortion at the end of a match when a Nexus explodes. Make that happen during his ult instead of just darkening the edges. Would be pretty awesome. And instead of him flying across the map to the target, what if he actually tore space and time with his claws, and jumped into a hole torn in front of him which appears at his target and he comes out of it near them and hits them. Doesn't have to be right next to the target, just some worm hole fun going on would be sweet and very time and space related. Would also keep him true to the teaser! Though if not tearing up time and space, I do like what previous people have said about the sound effects similar to those in the Transformers clips.
Also, sounds. Some of what he says is still hard to understand. I am not sure what he currently says at the beginning of his Ult, though it sounds like "Oblivion". Again, not sure because it isn't clear. Love the emotes though. His dance and joke are some of my favorites out of all champions.
And just to go back to his visual appearance, I saved what I think is the most important part for last. Since a lot of people compare his color scheme to that of Battlecast Cho and the other mechs (black/dark grey with red) simply change the red particles on his skin and abilities to a light blue. Not only does blue seem more of a spacey color, it would distinguish him from them as well as fit the overall theme of Nocturne better. We see blue colors present in quite a few of his skins and honestly it has that chilling creepy feel to it. Think that would just be one awesome change. Or even a purple-ish, though blue might look better. The designers should just play around with the colors and compare the blue to red and they might be pleasantly surprised.
If the skin was a theme and included an icon/banner I would definitely pay full price for it with some extra tweaks to what he currently is on the PBE.
Thanks for the time to read this and keep up the awesome work!
Edit: Another possible passive visual effect which would be completely rad and tie in with the skin's theme of space and time would be if he was trailed by faded images of himself. If you have played WoW or know how the Ethereal moved in it, that is kinda the effect I am talking about. Just snapshots of his past movements which trail slightly behind him, are transparent, and quickly fade away.