Actual implementation found in:

• HeartOfDarkness.java - Contains the HeartOfDarknessBuff class

public static class HeartOfDarknessBuff extends ArtifactBuff {
	@Override
	public int icon() {
		return BuffIndicator.DARKVEIL;
	}
 
	@Override
	public String name() {
        return StringsManager.getVar(R.string.DarkVeilBuff_Name);
    }
 
	@Override
	public String desc() {
        return StringsManager.getVar(R.string.DarkVeilBuff_Info);
    }
 
	@Override
	public int defenceProc(Char defender, Char enemy, int damage) {
		int defenderPos = defender.getPos();
		int spiritPos = defender.level().getEmptyCellNextTo(defenderPos);
 
		if (defender.level().cellValid(spiritPos)) {
			SpiritOfPain spirit = new SpiritOfPain();
			spirit.setPos(spiritPos);
			Mob.makePet(spirit, defender.getId());
			defender.level().spawnMob(spirit, 0, defenderPos);
		}
		return damage;
	}
}

• This buff is part of the Heart of Darkness artifact
• Name in game: “Dark Veil” (not “Heart of Darkness”)
• Effect: When the owner takes damage, summons a Spirit of Pain nearby
• Icon: BuffIndicator.DARKVEIL
• Implementation: defenceProc method triggers when taking damage

None found - implemented purely in Java

<string name="DarkVeilBuff_Name">Dark veil</string>
<string name="DarkVeilBuff_Info">A veil of darkness surrounds the owner, summoning a spirit every time damage is taken.</string>

This entity is implemented in Java, no Lua script exists