"… When monsters die, their dust will be spread over what they loved. An object that symbolizes their existence. That object will be buried… And in such a way, their soul be able to rest, within that object, and the earth." — Book about Monster Funerals, Deltarune Chapter 4
Thinking about The Scientist series by @talkingsoup and what Deltarune shows us of monster burial traditions.
Thinking about how Sans was never able to truly mourn Gaster at first due to memory loss and Gaster not actually being dead. But by the end of the series, after everyone gets their memories back, after Gaster passes on for real, Sans can finally grieve.
Thinking about how burials include a photo of the monster and an object of significance that represents them. Thinking about how serendipitous it is that Sans owns both of those things, what may very well be the last physical traces of Gaster left in all of existence.
Thinking about how Gaster didn't leave any dust behind to properly complete the ritual, to "connect his spirit to the divine" as Father Alvin puts it, but it's a thoughtful gesture nonetheless.
(Gaster never really believed in that sort of stuff, anyway.)
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In my head, the story behind this art would go something like this: at some point post-HTSTW, Asgore and Toriel are tasked with overseeing the transfer of the Underground's gravesites to a new cemetery in Outside. This creates the perfect opportunity to finally make proper burials for Gaster, Betas, and the followers, whom everyone now remembers and misses.
Asgore and Toriel end up asking for Sans's input on what should be included in their old friend Gaster's gravesite, since Sans was the last person to truly be close to Gaster before his death. Sans doesn't really think he's the right guy for the job; Asgore and Toriel knew Gaster for much longer, he points out. In response, Asgore recounts a newly-remembered memory of a phone call he had had with Gaster, long ago now—one about a promising new hire—and the way Gaster had sounded...happier, more hopeful, than he had in a long time. This is enough to finally convince Sans, who agrees to spend some time thinking about it.
After some rumination, Sans decides to go back to the Underground to retrieve the last remaining photo of Gaster that he had left behind in his old basement, along with several other sentimental odds and ends he wants to take back home with him. The memories associated with these items aren't as painful as they used to be, and he no longer wishes to let this stuff rot away, abandoned and forgotten. Sans doesn't go alone, though—Undyne accompanies him to ensure a safe journey there and back, and she expresses her surprise and confusion over Sans going so far out of his way to do something nice for Gaster, let alone wanting to be involved in his burial at all. Sans simply says that it's complicated.
Back on the Surface, Sans and Papyrus go to a human store to get the photo professionally framed. Sans doesn't really know what he's doing, so Papyrus helps him pick one out, asking Sans what type of person Gaster was and what kind of frame he would've liked ("Something high-quality, but not too ostentatious. He'd never forgive me if I cheaped out.")
Sans also decides to part with the chess piece that Gaster gave him, but not before giving it a fresh new coat of paint to cover up the wear and tear from all the times he fiddled with it in his pocket.
There's a funeral service for Gaster, Betas, and the followers, and Sans gives a eulogy for all of them. In typical Sans fashion, it's a mostly serious and sincere speech punctuated by some actually tasteful jokes here and there that get teary chuckles out of the crowd.
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Additional commentary:
Your standard king chess piece is normally topped with a cross, so I turned that into a similar, but different shape: the four pointed star associated with Determination. I wanted the chess piece to feel unique to Gaster, to make clear that it was special and not your average, everyday, run-of-the-mill chess piece.
I imagine in the Undertale universe (specifically on the Surface in the post-pacifist timeline) it is common for monster graves to be located above the ground rather than below the ground since monsterkind has so much trauma and baggage associated with being trapped underground. The graves would probably look something like the SOUL coffins that were interred at Asgore's castle, or like tombs. Gaster's grave would be no exception, seeing as how he toiled for over a thousand years trying to find a way to escape the earth, not go back to it. I think just pasting the Deltarune version of monster burials into Undertale with no alterations would mean ignoring the differences between the two universes' histories and would be symbolically questionable as well.
The object representing Gaster doesn't have to be the chess piece—that's just what I went with for this art since it seemed like the obvious choice for an object associated with Gaster that is instantly recognizable, highly meaningful, and can be found in the fic itself. Plus, since it contains some of Gaster's essence, it would make for a suitable substitute for his dust which probably no longer exists. In truth, however, I think I'd rather Sans not part with the chess piece permanently like this, especially since I headcanon that the reason why Sans seems to keep it with him in his pocket everywhere he goes is because he wants to share his Surface experiences with Gaster, at least symbolically. The chess piece being all by its lonesome stuck in one place would go against that idea. As for what item could be used in its place, I'm not sure. Something sciency that represents Gaster's passions and legacy, perhaps.
That being said, with regards to "their dust will be spread over what they loved", in this case I imagine the chess piece representing the time Gaster spent with Sans playing their chess games, and that being what he loved.
I think Gaster's dust would simply not be included in this ritual since by all accounts it shouldn't exist anywhere, but here's a thought: what if Gaster saved some of his dust from his near-death experience (maybe for research/experimentation purposes) and Alphys finds it hidden away after one final sweep of the True Lab? I don't think that would realistically happen (Gaster's erasure probably would have destroyed it anyway) but if you really wanted Gaster's dust to be included in his burial somehow, that's how I'd rationalize it.
As for Betas and the followers' graves, I imagine Alphys and their families were in charge of those.
