In the years that I had covered video games analysis on different websites that are defunct on their vast majority, minus one which is Adventure Classic Gaming that is still active and it was the first one I ever wrote for. I have to say, I never saw something as Undertale, the game is just retro and perfect; it offers a unique experience that is rarely replicated "a great story, a gameplay that is easy to understand but also it offers a cool soundtrack" I purchase it the game as I mentioned, when it was launched to the public, but it took me almost 3 years to finally playing it; not because of any external factors but I didn't had the same enthusiasm to play anything for almost 7 years.
There are some metaphysical concepts, mostly related to Buddhism in which there is a reincarnation path, but also there are more complex cosmological precepts such as that multiple realities can co-exist within the multiverse, making each reset of the game just another version of an infinite numbers of scenarios in the multiverse in which only a few characters are aware of the experiences they are going through, such as Sans or Flowey, and even Frisk as they can recall how many times they had been killed.
The fight Frisk has with Asgore, they can recall how many times Asgore has killed them, on my neutral route Asgore killed me twice, and Frisk mentioned it; but not only Frisk can recall the times they had died, Sans keeps track albeit he can't recall the full extent of everything but what decisions the player took to kill or spare the monsters
What it got my attention, is also how much you as the player can do to alter the fabrics of existence, but also how much it follows you to the different realities; imagine how fantastic would be just to jump from one level of existence to another, in which you as a person are just going to experience variations of your personal history but not as harsh or drastic, you can essentially re-live your life or take the place of your other self and live his existence.
The times I played Undertale, that question popped in my head, what if my actual life is just one of the many I had lived because through will power I just woke up in a point in history before my impeding doom? You could essentially be able to re-do your mistakes but also you would be trapped in an infinite loop, where happiness doesn't exist and you are at the mercy of the cosmos, the mercy of a higher deity that makes you travel in an endless cycle of misery.
The game ending can be extremely bittersweet, because it doesn't matter if you save or kill everyone; nobody can't save you and the only individual that shares the same destiny with you is a homicidal plant, that was given life thanks to the essence of a child that was murder. The more I think about the ultimate fate of the characters, it makes me sad, it brings me desolation to my heart that there is no happy ending, just despair.