THE KNIGHT unites!
The Knight has low walk and air speed, but very fast attacks. Its smash attacks, have surprising reach and are quite strong when charge, and its down smash even moves it forward. It has a SOUL meter next to its portrait, which fills with a little SOUL every time an enemy (or assist trophy) is damaged with its nail, and only the nail. The meter shows has small visual changes when it passes 33%, 66% or reaches 99%, to make it easier to read. Using down-air on enemies will give the Knight a little upwards momentum. Rolling moves it further than most other characters, and the first forward air-dodge it does will make it dash forward that same distance.

UP B: HOWLING WRAITHS
The Knight unleashes a white barrage of spirits above it, acting like a large, powerful explosion. The edges do significantly less damage than the center. If the Knight has 33% or more SOUL, 33% SOUL is spent to turn the move into the more powerful Abyss Shriek, which is larger, stronger, and has black visual effects.

SIDE B: VENGEFUL SPIRIT
The Knight launches a large white spirit forward, which travels at a decently high speed but doesn't go extremely far. If the Knight has 33% or more SOUL, 33% SOUL is spent to turn the move into the more powerful Shade Soul, which is stronger, moves faster and further, and is black.

NEUTRAL B: FOCUS
The Knight stands (or falls) in place, a white aura glowing around them while SOUL slowly drains until the button it let go, or the Knight is damaged by anything. When and only if 33% SOUL has been drained without interruption, the Knight will immediately heal for 10% damage. If this move is used while the Knight has less than 33% SOUL, nothing will happen, but no SOUL will be drained either.

DOWN B: DESOLATE DIVE
If grounded, the Knight jumps up a bit before doing this. The Knight drops down at high speed, damaging anyone it makes contact with on its way down, with no way out. When it hits the ground, a powerful explosion erupts around it, damaging and launching enemies. Like Howling Wraiths, the move is significantly stronger at the core than at the edges. If the Knight has 33% or more SOUL, 33% SOUL is spent to turn the move into the more powerful Descending Dark, which makes the explosion stronger, larger, and destroy projectiles, and has black effects throughout the move.

FINAL SMASH: GOD OF GODS
The Knight's mask breaks, revealing the Shade underneath. A void tendril whips forward out of it, grabbing any enemies in front of it and pulling them into its body. Next, a short cutscene plays of the targeted characters waking up on a nondescript platform, while a giant void being (https://youtu.be/y3navOsvDCw?t=2370, appears at 39:35) appears from the surrounding darkness. It closes in on the screen, fading it into darkness. The screen fades back to the battlefield, and the targeted enemies are launched violently out of the Knight's shade while its mask repairs itself.

I took a few liberties. In the game, magic requires SOUL to be used at all, and the stronger versions are just upgrades that you find. Also, the mask never repairs itself. It's not really explained how respawning works in-universe, actually.