We are but dust and shadow

Pen is quiet as he thinks on your question. It's painful to admit, even if only to himself, that he didn't notice his brother's descent until it was far too late. There is only one way for monsters to tread the path his brother now follows—and there is no going back.

He doesn't know when Umbra started to lose his grip on Hope, when his brother started to seek an escape in the fleeting sensations of humans' energy, in their emotions, to drown out what was likely his own apathy and despair. Pen regrets how naïve he had once been. Perhaps if he had been more aware, more attentive, he could have stopped his brother's corruption.

One look at his brother, at the glow of his single socket and the magic reacting and seeping from between his bones, Pen knows it won’t do his brother any good to be further worked up. He intercepts you.

“Human,” he starts, his ethereal voice pleading, “I am afraid that with things as they currently are, such a choice would be made in folly. My brother…” He pauses, taking a moment to gather his thoughts, to put his own emotions to rest. “Nothing good would come of offering yourself up to him like that. He’s… Sans has not been himself for a very long time. Please do not tempt him.”

Unable (or perhaps unwilling) to conceal his hunger at the mere thought of such a tantalizing meal, magic begins to pool and drip from the corner of Umbra’s maw. As the shadow grows ravenous, his magic reacts, flaring in the spaces between his bones—never mind it hasn’t been long since the last.

Umbra’s gaze drifts to you, a single thought in mind.

[[Most monsters don’t remember life prior to being locked away, so most have nothing to really miss. They can also see the sun and moon when interacting with the human world, but it looks different to them from their plane of existence. Those that remain from before their imprisonment, however, do miss it.

As seen below, the monsters’ and humans’ planes exist parallel to one another and do not intersect. 

Image text reads: The humans and monsters live in separate planes of reality.

Humans cannot interact with the monsters’ plane, nor are they able to perceive it*, but monsters are not quite as limited. Monsters can view the human realm from theirs, though it is colorless—monotone.

They can go out in light, as they’re not really in it. Their view of the human plane is something like a “mirror”.

* Every now and again there are humans able to see monsters. It is incredibly rare, but there are some humans that have been able to even see into the monsters’ plane.

To monsters interacting with the human world, its appearance is quite similar to their own.

Regardless of the time of day in the human world, things would look similar to the above. There is no color and the farther away something is in the distance, as well as the less amount of light there is in the human world, the darker and more a silhouette it is.

Strange... is that a glow on Umbra’s face? ...an eye?

The shadows shift and swirl before you, the writhing tendrils looking suspiciously like hands. They reach towards you, clawed digits splayed, and you find yourself thinking they look as if they’re expecting something from you.

You have succeeded in flustering the skeleton. He was not expecting this, and still more than a little confused after the earlier smooch, but not wanting to risk embarrassing you by turning you down, he obliges.

Like all previous contact with the shadows, the hug feels... strange. The sensation does not linger long enough to warrant any concern, however.

[[ I'm going to assume you meant Umbra ]]