The Roaring Knight Boss

Spoilers

The Chapter 3 boss

The Knight is the figure behind the Dark Fountains. In Chapter 3, after the battle in the Cold Place, it strikes Tenna down and tries to take Toriel. Susie stops that, and the party fights. If the party "wins," the form stutters, Susie swings, a shard breaks off the Black Knife, and the Knight still swoons Susie and Ralsei and forces Kris to kneel. The scene then continues as a loss. Undyne interrupts the next grab and is taken instead.

The wiki's clear condition is violent. S-Action and R-Action do nothing and are removed after one use. HoldBreath speeds the SOUL for the whole battle, and using it again changes nothing. HP has to fall below 80%, which the wiki counts as 1460 damage, before the full-screen Roaring attack. Any successful hit after the line "The enemy suddenly let down its guard!" ends the battle.

Jevil is the Chapter 1 secret boss who talks about the Knight's hand. Spamton NEO is the Chapter 2 secret boss, and this Scratch project is a remix of that battle's engine.

Spoilers

How the attacks come

The Deltarune Wiki gives the Knight a fixed list. The first three phases are cycles of five attacks. Phase 4 begins once HP is under 80%, at the end of the current attack. Flavor text usually names the next attack. Holding cancel slows the SOUL to half speed.

• Stars: a purple cone fires six-pointed stars that grow, reverse, turn red, and burst into shards. Later stars burst into six shards, then into two shards that home in. The star's hitbox is the center dot. The light before the burst shows which way the shards go.
• Tracking swords: a sword follows the SOUL from a cardinal or diagonal direction, turns red, and leaves a slash. There are 10. A later version adds a circle of swords around the SOUL. Another version is the same swords, faster, without the circle.
• Box splitter: the board is cut and both halves shove apart while diamonds spray from the cut. The slash deals 66% of max HP, or 33% with the Shadow Mantle, and it does not reduce anyone below 1 HP. Later cuts can change axis, then add diagonals. The flavor lines talk about the north wind, then the north and east winds, then a tempest.
• Sword tunnel: blades slide in from above and below, then turn and fly across on red aim lines. One version rotates the angle. One version narrows the gap. Invincibility time on this attack is very short.
• Rotating slash: red lines spin, freeze, and cut. The count of lines climbs through the attack. The last version adds a spinning set of cuts at the center. These hits land on the whole party.

Phase 4 still shows the twisted-heartbeat slash if you have not seen it, then a turn where the Knight only charges. That turn is the heal window. DEFEND does not blunt the Roaring attack: the wiki says defense, defending, and element resistance are ignored. Stars pull inward, spiral out, return, and burst into six, and a slash comes down the middle. Bullets deal 40, or drop a character to 1 HP if they had between 2 and 40.

The battle frame on this page shows the star bullets around the red SOUL while the Knight points from the right. Bad_Time_Trio1's changelog says several of those patterns were sped up, including the star attack, the swords, the box slash, and the final attack.