timeline

It's occurred to me I haven't shoved this on this blog yet. Seeing as I can't link to a good portion of it since it's on a site that can't be linked to here - for all its lack of relevancy, unncessary length, and without further adieu,

Before the War
👉 ☠♱☹☹ was born to a pair of monsters (undead subtype: a wight* and a ghast) in a small mixed monster-human village. His parents were undertakers/funeral directors, performing their services for both monster and humankind.

👉 He spent a portion of his early teen years studying under the village's alchemist before the growing unrest reached the village.

During the War
👉 While he was old enough to fight at this time, his father made him promise to take care of his mother and sent them both off to a monster refugee camp at the summit of Mt. Ebott.

After the Barrier was erected
👉 ☠♱☹☹ lived with his mother and continued his pursuit of alchemical knowledge. They settled in the newly founded Home.

👉 Nothing significant happened. Scientific knowledge progressed. He switched from alchemy to chemistry to biology to physics to quantum physics and combinations thereof, keeping up with breakthroughs and discoveries while one day hoping to make his own.

👉 His mother eventually relocated to New Home. ☠♱☹☹, on the other hand, relocated to the laboratory in Hot Land to study under the as-of-then current Royal Scientist as an assistant and an intern.

👉 He eventually took over as Royal Scientist.

👉 As science progressed, technology progressed. Rudimentary methods of generating electricity were developed. It was around this point that Alphys joined the Hot Land science team; she quickly became his right hand man. Or... Woman. Right hand lizard lady. You get the gist.

👉 Development began on the CORE. Asgore inquired about a scientific means of breaking the Barrier; development also began on DT extraction and the study of the human SOUL.

👉 The first CORE prototype malfunctions. The explosion kills one assistant and injures ☠♱☹☹. The incident leaves wounds that never fully heal, both physically and mentally.

👉 A few months after the first incident, Sans joins the Hot Land science team as an intern. Before long, he becomes an assistant.

👉 The second CORE prototype is activated successfully. Work begins on a larger version that would ultimately become The CORE.

👉 Alphys and Sans become ☠♱☹☹'s 'favorites.' They are both candidates to become the next Royal Scientist, whenever he decides to step down.

👉 An attempt is made to develop more than just a working relationship with Sans. Unknown if successful.

👉 Asgore sends a human to the Hot Land laboratory for the purpose of study. It's the second fallen human. The DT extraction process is tested on this human, proving successful--but, ultimately, fatal. The SOUL is captured and sent back to Asgore. A general sense of unease settles over the science team. The next two fallen humans are sent to the laboratory alive to be studied, extracted from, and dealt with; after a talk with the King about how this is lowering morale, the rest are sent already deceased.

👉 Development begins on the first attempt to break the Barrier: the 'Gaster Blasters.'

👉 The Blasters fail to break the Barrier. They do, however, prove to be effective as weapons. ☠♱☹☹ decides not to decommission them and keeps them, just in case.

👉 The CORE reaches completion. Activation is successful. The Underground has a consistent source of power.

👉 Experiments with DT begin. These experiments are delegated to Alphys.

👉 An attempt is made to create artificial human SOULS for a second attempt at breaking the Barrier. It fails. Asgore calls off the scientific approach to Barrier-breaking.

👉 ☠♱☹☹ begins spending more time in the CORE. There are rumors about him talking to it. He takes over as its sole maintainer, moving those whose duty it was to maintain it prior to other projects.

👉 His mother falls down. She is sent to the Hot Land laboratory and put under Alphys's care.

👉 We all know the result of the DT experiment on fallen monsters.

👉 The stress and tragedy of life begins to culminate into something that gets to be too much. ☠♱☹☹ does not leave the CORE facility for a month. Then, he jumps.

👉 The timeline continues as normal. ☠♱☹☹ is declared deceased and treated as such. Initially, his existence is not erased. Initially, he is just that: dead.

👉 Frisk falls. A neutral route begins. At the end of it all, they Reset.

👉 The Reset triggers data retrieval. There is an error in retrieving ☠♱☹☹'s data. The universe compensates. His existence is erased and Alphys takes his place. The Room is created to confine the error caused by ☠♱☹☹'s incomplete retrieval. What little of ☠♱☹☹ is retrieved is in that Room. What cannot be retrieved is substituted with a substance called "Void." The Room is only present within his home timeline once, but another Reset occurs before Frisk reaches it. It is never opened. It disappears.

👉 The door to his Room begins to appear in entirely different universes under the subsection of Undertale within the ever-expanding multiverse. Every Reset shunts his door into a different universe. ☠♱☹☹ gradually learns about the state of his existence. ☠♱☹☹ gradually learns about the state of existence, in general. It's around this time that he takes on that name, "☠♱☹☹," to differentiate himself from other Gasters.

👉 He develops an interest in returning to his home universe. He develops an interest in 'the simplification of the multiverse' in order to make that even the slightest bit possible.

👉 He begins development of machines that can, in theory, manipulate the very fabric of the worlds he's in on a quantum/code level. However, the machines are too large to fit into his Room. He has to start from scratch each time. The going is slow. Fortunately, he has nothing but time.

👉 He begins experimenting in easier ways to make universes unviable. One of these ways is talking those universe's inhabitants into "joining him" by jumping into the CORE.

👉 Morality degrades with time and desensitization. If a monster in one universe dies, they're still alive elsewhere. It doesn't matter. Life is, quite literally, a game. He can prove it mathematically. He could provide tangible proof if given enough time within one universe to build one of his machines. That is rare, however. It may have happened only once at this point. His Room tends to appear shortly before or shortly after Frisk or some equivalent has fallen into the Underground: Resets happen too often to progress far enough.

👉 Present time.
 

* I use “wight” to refer to “undead of mixed lineage." Gaster was born looking enough like a skeleton that his parents thought they could be clever and call him “Wingdings Gaster” both as a pun and so he’d fit in with other fontly-named Skeleton monsters.

His skin and underlying musculature grew in over time as he approached his teens, much to his parent’s chagrin.