EntropyTale's main differentiation from Undertale is that the mechanics of RESETing are vastly different. When the protagonist RESETs, they arrive in a new, slightly different, and slightly worse, world. Every one of these worlds are referred to as "iterations" or "Sub-AUs", or else a different, specific name given to it. Each of these iterations can be treated either as part of EntropyTale as a whole, or an individual AU. Note that this effect works only when RESETing, not LOADing (Frisk cannot LOAD to a previous iteration after RESETing; File is inaccessible.)
The first iteration was one of happiness, in which humans and monsters got along on the surface. The war between humans and monsters never happened in this iteration. However, when Frisk goes to play on Mount Ebott with their friends, they accidentally trip down a hole. Their friends go off to get help. Frisk can travel through the empty Underground, checking things. If they 1) check the blank flower at the beginning, or 2) reach the end of the cave, they are given the option to Reset or Do Not. If Do Not is chosen, then nothing happens, and the tale ends. If they choose "Reset", then they appear back at the beginning. The list of iterations can be found below:
Note: It may be that Frisk's doing a Genocide run would stop the nightmare, as Flowey claimed it would. However, he may just be teasing them.
Iteration 1; Home
Frisk's home timeline, as described above.
Six dead Fallen Humans can be found Underground.
Seven dead Fallen Humans can be found.
Seven dead humans can be found, as well as the dust of several monsters.
A group of hostile monsters is found, the first major divergence. It is revealed that long ago, humans demanded that monsters put their prisoners Underground in distrust and fear of them, and seal them down with a barrier.
(It. 6) Many more monsters, many falsely accused, could be found Underground.
All monsters are trapped Underground. King Asriel, Princess Hope and Gaster, the Royal Scientist as well as Undyne, the Chief of the Royal Guard do not attack Frisk, as the monsters haven't instated an anti-human policy.
Frisk finds Hope Dreemurr, Asriel's daughter, in the Ruins, and learns that Asriel was forced to implement an anti-human policy. Hope now strongly dislikes her father, and the monsters attack Frisk.
Gaster is no longer alive. No Royal Scientist replaces him. Sans and Papyrus can be found in Snowdin. Hope is learned to have died before the events of this iteration.
(It. 10); Flowey appears outside the Ruins and at the End. Alphys is the Royal Scientist. Mettaton now has a body and is a star. Asriel is no longer King, and Toriel inhabits the RUINs and Asgore takes the reign as King. This iteration is what we know as UNDERTALE by Toby Fox, except for minor differences in dialogue.
It is revealed that Flowey can remember each timeline Frisk goes through in which he exists. Toriel seems less confident. Asgore is much easier to defeat. Alphys is very shy and does not appear in the Pacifist Route.
Toriel has rebelled against Asgore, and has a following of monsters which protect her against him.
Asgore and Toriel have conflicting nations, Toriel being the Queen of the Kingdom of Free Monsters, and Asgore being King of the Kingdom of Monsters. On Toriel's side is Mettaton, Sans, Papyrus, and Alphys, and on Asgore's side is Undyne, the Royal Guard, Muffet, and Flowey.
(It. 14); Asgore is left to rule, but has much of the Underground under quarantine from a virus.
The Underground is full of dust. Very few monsters have survived the virus, including: Napstablook, Grillby, Mettaton, Alphys (in self-preservation), Annoying Dog, Flowey, Temmie, and likely a few others.
(It. 16); The same as the last iteration, however, this iteration begins the trend that monsters' bodies do not turn to dust when they die. Bodies lay everywhere. Flowey comments that this seems like it happens "only in spite of you" and that the universe is mocking the protagonist.
Flowey was able to get the Six SOULs and now has complete and utter control over the Underground. He is like a literal god, able to manipulate reality, bring people back from death, and so forth. Begins with an illusion by Flowey to mock Frisk.
(It. 18); Flowey still has the Six SOULs. However, in this iteration, Flowey has become King of the Underground and is worshipped as a god. Flowey takes refuge in Papyrus' empty husk. Sans, Alphys, Undyne, Mettaton, and others resist Flowey's rule in the Hotland Labs.
(It. 19); Toriel, Asgore, Muffet, Undyne, Gaster, and Flowey each have their own individual nations in the Underground. Monsters in general are more noticeably aggressive than previous iterations.
(It. 20) The Kingdom is united under the rule of Asgore, who is a fierce dictator and has instated tyrannical laws, ruling over his citizens with an iron fist. His nation is a totalitarian society reminiscent of fascist or communist governments, despite being a monarchy.
(It. 21); All monsters have their alignments switched. Comparable to Underfell, and rather similar, though minor differences.
(It. 22); The monsters seem much less inherently volatile, curiously, and are back to their usual state. However, many monsters in the Underground are infected by a virus caused by the escaped Amalgamates, making many monsters melting zombie-type creatures.
(It. 23); Monsters are infected by a virus which makes them similar to zombies, although they are not dead in such a state. They simply hunger for flesh. It is comparable to HorrorTale and AxeTale, however significantly different in some ways as well, including there being a small general plot revolving around groups that are immune and/or survived.
(It. 24); Monsters are beasts, and not the usual intelligent species seen in other iterations. They are creatures that follow mostly primal instinct, though many are kind and have morals, as well as understand language. Some can even speak.
(It. 25); Monsters are monsters in the human meaning of the word. They are the true monster equivalent of the creatures they represent, often bent on killing, though many do not wish harm unless provoked, and nearly all are deadly if done so.
(It. 26); When Gaster fell into the Void, an explosion occurred. Instead of Gaster falling into the Void, the Void fell into the world. Gaster now lives in a calm, structured and, albeit vacant, pocket universe. The universe he fell from, however, is in chaos, glitching and ebbing in and out of separate timelines, getting more and more unlikely possibilities over time, eventually even mimicking certain alternate universes in the EntropyTale continuum. Inspired partially by Errortale.
Extras/Alternate EntropyTale AUs
Every time Frisk RESETs, they end up in a different, pre-existing AU.
Every time Frisk RESETs, they end up in a new Swap AU (note: swaps do not have to be canon to another, pre-existing AU to occur.)
Every time Frisk RESETs, they end up in a better world.