Whither Dost Thou Wander?

Personally, I think he would have used his shapeshifting ability mainly as a way to break people's spirits. Nightmare can delve into the minds of people whose resolve has weakened and read the surface of their memories almost as if he were brushing against them. Most of the time, he uses negative stimuli to draw out the memories he needs, confirm them, and then copy them for his own use.

However, reading memories that way becomes difficult when dealing with someone who has exceptionally strong mental fortitude or very strong positive emotions. In those cases, he first has to provoke negative emotions somehow, and usually he solves that by creating terrible situations. It's rare, but when someone is difficult to reach in reality, he may even interfere through dreams instead.

He's very skilled at encouraging and manipulating the emotions of others, so even the smallest trace of negativity can be amplified and shaped however he wants. To him, other people's emotions are almost tangible, and manipulating them is something he's been doing for centuries.

Surprisingly, simply influencing and manipulating a handful of people can produce a considerable amount of negativity. Perhaps because their lives are finite, mortals possess an astonishing amount of emotional potential. Even so, Nightmare consumed so many apples that the sheer amount of negativity he requires is enormous, and satisfying that demand still takes countless victims.

Nightmare actually enjoys this process quite a bit, and after the chaotic hatred of his early years settled down, there was even a period when he stopped killing altogether and spent his time doing only this. Almost like becoming completely absorbed in a single hobby for a while.

However, he doesn't rely on this method very much anymore. It's efficient, but it's also extremely tedious. Nightmare has already broken far too many people, and unless someone is particularly interesting, he doesn't really feel like putting that much effort into them anymore. Put simply, he's just tired of it.

The reason is different from when he first entered the multiverse, but these days Nightmare generally prefers killing people outright rather than carefully manipulating their emotions. The dead don't produce negativity, but they don't produce positivity either. The people left behind are forced to face terrible situations, and the fear, anxiety, anger, and grief that spread from person to person are an easy source of negativity.

For that reason, current Nightmare doesn't spend nearly as much time carefully manipulating people's emotions as he once did. Naturally, he has far fewer opportunities to make use of shapeshifting as well.

On top of that, Nightmare doesn't particularly like his own appearance, and he tends to act as though he doesn't care about it at all, almost as if he's trying to hide the fact that he does. He's spent so long pretending not to care that I think it almost became genuine at some point.

The "Sans-type" form Nightmare currently maintains is actually one of the most common skeletal forms in the multiverse. In the past, he kept a form that looked much closer to the one he had as a child, but after a certain event he adopted a different appearance. He judged this form to be the most common and least remarkable one available. At the time, he desperately wanted to convince himself that there was nothing special about him anymore.

That said, after keeping the same appearance for such a long time, he's grown accustomed to it as well. It wasn't all that different from his previous form, and since he uses his tentacles far more often than his hands, having somewhere to store those "unused extremities" was actually convenient.

At this point, I think Nightmare's perspective on a body is probably a little different from most people's. He still uses his hands for plenty of ordinary things out of habit, but after spending so long existing as something fluid, individual body parts probably don't carry quite the same weight for him anymore. Since he's essentially a mass of liquid, the sensations he experiences through the forms he imitates are likely very different from what most people would feel.

Oh, and this is just something that came to mind while we're on the topic... Before Dream woke up, Nightmare was actually around the same height as Classic Sans. It wasn't exactly the same since he was mostly just eyeballing it, but it was pretty close.

But after Dream woke up, and sometime after realizing that Dream had become much taller, his form somehow ended up being just a little taller than Dream's. As for why... well, who knows. At the very least, I don't think he even noticed the change himself 😏

Anyway, for all of these reasons, current Nightmare has very little reason to use his shapeshifting ability. Since he's fundamentally made of liquid, I don't think different forms feel all that different to him in the first place.

As much as I enjoy imagining things like that, I also don't think he has much reason to take animal forms... If his goal is simply observation and gathering information, he can just hide inside shadows instead. Unless someone is making a deliberate effort, it's impossible for there to be light without shadows.

Still... personally, I do think there was a time when he first became fully aware that he possessed a shapeshifting ability and found it genuinely fun. I can imagine him testing his abilities with a child's curiosity and using them to play little tricks. In the process, I think he would have learned their limitations as well.

For example, even if he created wings, he wouldn't actually be able to fly. No matter how much he changed his shape, the liquid that made up his body would still contain an incredibly dense amount of magic, and liquid always falls. Negativity tends to settle at the bottom as well. Nightmare probably felt somewhat conflicted when he first realized that. Even after gaining all that power, it probably felt as though the sky and the light still weren't things that were meant for him.

But I think he eventually came to a different conclusion. If he couldn't have those things, then he could simply drag them down and destroy them instead. That way, nobody would even know what it was that he could never possess. After all, something nobody knows about is practically the same as something that doesn't exist.

Well, I feel like my answer ended up being a little vague in the end. Still, I like the idea that Nightmare is fluid by nature, both literally and figuratively, and because of that he doesn't become particularly attached to any specific form. Whenever he takes on a different appearance, there's usually a practical reason behind it. At the same time, when he keeps a form for a very long time, it makes me feel as though there's some kind of fixation hidden there.

I always find it interesting how Nightmare keeps trying to manage his emotions through logic. Emotions aren't really things that can be fully defined, so no matter how much effort he puts into it, all he does is accumulate more contradictions. That's part of what makes him so interesting to me.