Amita Amitabha

Transmigration into One.
Amita Amitabha is Saver’s major Noble Phantasm. It is the final and greatest stage of his minor Noble Phantasm, Chakravartin: Turner of the Wheel.
According to the reincarnation ideology centered on the Hindu God of Creation Brahma, all humans are a reincarnation of Brahma, and thus the entire human race is part of the same individual. (The Hindu reincarnation and transmigration timeline is sometimes ignored in our (Type-Moon) works, so don’t be silly and nitpick.)
According to this ideology, all humans will certainly attain nirvana. (become a Buddha)
This is because all humans will ultimately attain enlightenment and transmigrate.
…Uhhh—Well I have no idea what I’m saying anymore, all this transmigration and immortality business isn’t easy you know! Wow! So anyways… (Remainder omitted
To cut a long story short Amita Amitabha saves mankind from all its pain and suffering. “Everyone will become one with me through the wheel of reincarnation.”
It deals heavy damage that no living thing, no human can take, but the protagonist isn’t alive in the first place (spoiler!) so he has a small chance of withstanding it, since the protagonist has yet to be afflicted by the suffering of life or the four noble truths.
If the production budget of EXTRA were 10 times larger, we would have included a gimmick where the protagonist would achieve enlightenment (gain strong convictions) over the course of the game and overcome this attack.1
When fighting against Saver we wanted to evoke the feeling that if players don’t win the battle in one turn, this “inescapable game-over attack” would activate.
By the way, in the game the actual damage is 5,670,000,000.

Translator's Notes
  1. ^ Attain enlightenment = become a Buddha = be able to survive the attack.
Fate/EXTRA Material: Encyclopedia of Fate/EXTRA