I was actually introduced to this artist by my girlfriend when she bought a print of the piece you see above back from a convention. I have a soft spot for Undertale fan art, so I immediately fell in love with the piece. After looking up the rest of their work, I happen to adore NightMargin's style.
I think part of what draws me to their work is that it seems that indie games are their focus. Besides a lot of pieces based on Undertale, they have a lot of art based on Hyper Light Drifter, which I found to be absolutely fantastic.
If you want to check out more of NightMargin's work you can check out their Deviant Art page.