For those who are not fans of DELTARUNE, here is a summary the Roaring Knight's powers and abilities
1. Extreme levels of raw strength and speed
The Knight has clashed with the main bosses and heroes multiple times, and struck them fast and hard enough to instantly knock them down to a state beyond normal recovery and before they could ever react. They are stronger than every other character in Deltarune, and are a threat to entire dark worlds. Even your strongest OCs, realistically and at best, are comparable or on par with the Knight. Additionally, they are tough enough for any normal strike from weapons to be basically meaningless.
2. Dark Magic and forming things from darkness
The Roaring Knight can create what seems to be just about anything by making magic things with darkness, such as swords that can fly egregiously fast, crystals that shatter into numerous smaller projectiles, bubbles of darkness that can hold and suspend people inside of them, etc. The Knights attacks are magic, and deal high damage to your very soul, with normal defenses such as armor doing very little against the Knights attacks. Only very specific magic armors can defend against the Knights powerful magic, and even still only mitigate damage rather than prevent it.
3. (Kinda) breaking the rules of magic systems
The Knight has sort of shown they possess to ability to somewhat defy the "laws" of their worlds magic and combat on a somewhat meta level, such as slashing down high amounts of damage to essentially make them nothing, defying the otherworldly power of "determination" (which otherwise should guarantee victory) to force you to lose, and things of that nature.
4. Cutscene magic (/j)
If a character attempts to battle the Knight in a cutscene, they are automatically guaranteed to lose to the Roaring Knight by some bullshit insta-kill they pulled outta their Roaring ass
Have fun and don't get into cutscene with them