Undertale Boss Battles Script

Few indie games have left as deep a mark on game design as Undertale (2015). Its boss battles aren't just fights — they are conversations, emotional crescendos, and puzzles wrapped in a bullet-hell shell. For modders, fangame developers, and coding enthusiasts, recreating that magic starts with one thing: the script.

An "Undertale boss battles script" refers to the code (often in GameMaker Language (GML) or Unity/C# for fangames) that controls the flow of a boss encounter, from dialogue to attacks to sparing mechanics.

ACT commands:

const actCommands = 
  "Check": () => showBossStats(),
  "Flirt": () => 
    if (boss.name === "Papyrus") boss.spareProgress += 30;
    showText("* You flirt. Papyrus blushes.");
  ,
  "Threaten": () => 
    boss.dialog = "* ...?";
    boss.spareProgress -= 10;
;

Spare condition:

function checkSpare() 
  if (boss.spareProgress >= 100 && boss.spareEnabled) 
    endBattle("SPARED");

---

Before we write a single line of code, we must understand that an Undertale boss fight is a state machine. A static enemy that repeats one attack is boring. A great boss script cycles through phases.

let dialogQueue = [];
function showText(text) 
dialogQueue.push(text);
typewriterEffect(text);

function typewriterEffect(text) 
let i = 0;
let interval = setInterval(() => 
if (i < text.length) 
dialogBox.innerHTML += text[i];
i++;
 else 
clearInterval(interval);
setTimeout(() => dialogBox.innerHTML = "", 1500);
, 30);
 Undertale Boss Battles Script

---

START Battle
  Load BossData (HP, Attack, Defense, Name, Dialog)
  PlayerTurn = TRUE
LOOP while (Boss.HP > 0 AND Player.HP > 0):
IF PlayerTurn:
Display "ACT", "FIGHT", "ITEM", "MERCY"
HandlePlayerChoice()
ApplyEffects()
PlayerTurn = FALSE
ELSE:
ChooseBossAttack()
StartBulletPattern()
ApplyDamageToPlayer()
PlayerTurn = TRUE
END LOOP
IF Player.HP <= 0: GameOver()
ELSE: SpareOrKillCheck()
END
 Few indie games have left as deep a

---

player_hp = 20 papyrus_hp = 150 mercy = 0 state = "PLAYER_TURN" selected_option = 0 menu_options = ["FIGHT", "ACT", "ITEM", "MERCY"]

Now, let’s script actual boss behaviors. Below are templates for three famous encounters. Spare condition: function checkSpare() if (boss