"Your Best Nightmare" intro riff is the basis for some motifs we hear throughout the game
Not directly an Asriel knight theory, but it's asriel knight flavored and certainly makes me confident he will have a significant place in the story, and imo it provides proof for AsrielKnight.
"Roots" is something a lot of the theorizing community has been speculating about since chapter 3 and 4. What none of them ever mention is that the "roots" motif in Digital Roots and Bit Roots, is a version/modification of the intro riff to Your Best Nightmare. It plays in two of the most mysterious locations in the game - Queen's basement before Spamton's fight, and the manhole dungeon before we fight the shadow mantle boss. It also seems to just be a part of a song, and is repeated over and over in its uses, similar to the intro riff to "Your Best Nightmare".
There's also a major key version of the riff - the Dark Truth motif. It appears first when Ralsei explains the nature of dark worlds, but is also in Neverending Night (ch4 credits theme), and notably plays while the knight is attacking the fun gang with walls of swords at the end of chapter 4.
I'm not sure how to explain the similarities here in a way understandable to everyone, and feel like to even people who don't read sheet music there are visible similarities, so I've highlighted the notes present in the "best nightmare" motif with a color corresponding to a same note, so hopefully people who don't read notation can understand. I've tried to use color similarity to portray musical similarity. For instance if there's an interval that's similar but not the same, the color will be slightly different. I've linked all these songs in the body as well so you can listen.
Overall, I'd say there is absolutely no doubt in my mind these are all directly based off of Your Best Nightmare. The major differences are Bit Roots and Digital Roots having some extra notes and ending on a perfect 5th instead of a major 3rd, and Dark Truth being in a major key and ending on a major sixth.
Susie's battle theme features a similar riff, but it changes significantly in the riff's second half. Like the Omega Flowey fight, Susie has a pre-battle intro featuring the riff (You Idiot/Imminent Death), then a full battle theme (Your Best Nightmare/Vs Susie). Her using this makes sense - her character parallels Asriel in a lot of ways. Asriel also readily accepts "lower realities" as real, praying for forgiveness after killing Yoshi, and like Flowey, Susie says she viewed herself as a "broken toy". There are certainly more parallels I won't get into here but, imo comparison and contrast to Susie's beliefs and motivations is an important aspect of an antagonist in this story, and her having a motif similar to Asriel's calls to mind these comparisons with him in particular.