DELTARUNE Glitches: 1 HP Light World
Kris, having 1 HP in the Light World.
This glitch is interesting, because it uses another one to work.
One may call that Glitch Stacking.
First, fight Queen, and get Kris DOWN before winning the fight. (with them still DOWN)
Then, do NOT heal them, even with SAVE. You might notice their HP is still negative. This is the 'first glitch'.
Defeat Giga Queen. Do note that the battle may have visual glitches. Do not worry about them, they do not change anything important.
Equipping Kris in the way that gives them the strongest attack power is optimal against Giga Queen, as you deal more damage in this battle depending on Kris' AT.
Seal the Fountain.
Open your Light World menu. Congrats.
For this, we must talk about the way battles work in this game.
At the end of a battle, if a party member has negative HP, they will be brought up to low health. Additionally, all TP is drained and converted to D$ using a certain formula.
However, some battles end... unexpectedly.
This is an out-of-battle Heal Prayer. Or, rather, three of them.
It is only possible in the first chapter of the game, from the POWER menu, if the player happens to have TP in the overworld.
Its healing value is always 100.
Current TP is saved at SAVE points.
Indeed, in the second chapter, even if you happen to have the TP, it will simply not work. It was 'patched'.
As stated earlier, TP is drained when a battle ends.
But some battles do not actually run the script for a battle ending when they do. Those battles can be said to "end incorrectly", and include: SusieVsLancer (first chapter), Queen (second chapter), Giga Queen (second chapter) and Spamton NEO (second chapter).
Each of those battles except Queen and Spamton NEO (basement) only have one party member, so you cannot end them with negative HP as you would simply die.
Basement Spamton NEO used to also allow for negative HP, but it was fixed by adding a check that sets the HP to each party member to 1 if it is negative.
This leaves Queen as the only battle where this is possible.
But of course, Light World HP is not the same as Dark World HP.
So it may still sound odd that this would affect it.
global.lhp = (ceil(global.hp[1] / global.maxhp[1])) * global.lmaxhp
if (global.lhp < 1)
        global.lhp = 1
if (global.lhp > global.lmaxhp)
        global.lhp = global.lmaxhp
This is the part of the code that decides on Kris' HP in the Light World when it is entered, from "GlobalScript_scr_become_light".
It does something very simple.
It first set your Light World HP to
(ceil(global.hp[1] / global.maxhp[1])) * global.lmaxhp
This may sound complicated if you are unfamiliar with this game's code, so to quickly break it down: global.hp[1] is Kris' current Dark World HP. global.maxhp[1] is Kris' current Dark World Max HP. global.lmaxhp is Kris' max HP in the Light World, which is always 20.
ceil() is a function that returns the 'ceiling' of a value. Meaning it is always rounded up, to the larger number. ceil(1) will give 1, but ceil(1.1) will give 2. ceil(-0.5) would give 0.
(global.hp[1] / global.maxhp[1]) will give the fraction of HP that Kris is currently at.
If this was then multiplied by 20, it would successfully make the HP proportional, with the ceil() function used to make sure that it cannot get rounded down to 0, as Dark World HP is more than twice as large and thus using the round() function may round to 0 if your HP is low enough.
However, this is NOT how it was coded.
Instead, the ceil() happens... on the proportion. Meaning, if Kris' HP is not negative or overhealed, it will always give 1. (or 0 if it is 0, but no party member can usually get to exactly 0 HP in Deltarune due to the way DOWN works)
Kris is the only party member who cannot be affected by the overheal glitch, so the only way to get anything else than 1 (20 hp) is to have negative HP, which, as it cannot be below their max HP in negative numbers due to the way DOWN works (sets to half of it), will always round up to... 0.
The rest of the code simply locks the value between 1 and 20, so it cannot go below 1 or above 20.
This is why leaving the Dark World with negative HP on Kris gives them 1 HP in the Light World.
Last note: eating the Box of Heart Candy will not kill Kris from dealing one damage, as it actually simply sets their HP to 19, rather than decreasing it by one. In this case, it actually heals them.