Yamaloka Naraka
Punishing Abode for All Ye Sinners
Type: Fortress
Rank: A
Range: 30
Maximum Number of Targets: 340
The great temple and prison of Yamarāja. It represents the domain in which Yama processes the souls of the dead, cleansing them of their sins and returning them to the reincarnation cycle with superior dharma than when they entered. A great fortress is erected, and the domain of the Death God is established. Within this four-floor domain, even the process of death can be reversed, as the dead must cycle through Yama's judgement before they are cast through the reincarnation cycle. It would be exceptional, but with enough mana, a new body could be constructed for such a wandering soul. That's the power of the God of Death.
Within this fortress, Yamarāja obtains a rank-up to every parameter, and negates the fame bonuses of other lifeforms. This is because the Underworld is a place of neutrality, free of any bias for or against others. In this domain, Yama's rules are absolute. Other gods have their Authorities diminished, and Servants experience a skill block, and Noble Phantasm releases are made impossible. An exceptional cheat-skill that could turn the tide of battle against anyone. Events and possibilities are culled down, and even Magic Circuits experience a pressure, reducing a magus' mana capacities by half. However, this effect cannot apply to those who have suffered death under certain circumstances. For example, those interred with lamps are granted the boon of light, and those who were charitable or who died in honourable combat suffer lessened versions of the effects. Within Yamaloka Naraka, Yamarāja gains the skill of
Familiar (Yama), allowing the free summoning of Yama-dutas, Yamarāja's personal aids who distribute punishment and reward to the inhabitants of Yamaloka. Each one is equal to a low-rank Servant, though their individual power relies more on the domain of Yamaloka. Outside the fortress walls, they can only exist for 30 turns before disintegrating, but within the walls, they have power over those who Yama is cycling through the hells of Yamaloka, scaling with the existential dharmic and karmic values of those inhabitants. Against those who are "Evil" with a long history of atrocities, they gain exceptional defensive and offensive powers, while those who are "Good" with virtue and charity can dismiss the Yama-dutas easily.
There's the special case of the few who have achieved Moksha, and escaped the cycle of reincarnation; such a person suffers no effects from this Noble Phantasm, as the domain of the Death God has no bearing on one who will not suffer reincarnation.
In the case of a catastrophic event, Yama may initiate a palace-scale Self-Destruct, flooding the entire Noble Phantasm with water. This is because of the accounts wherein Yamarāja's domain is in the south, at the bottom of the infinite waters that rest under the universe.
Dharma Amitābha
Domain of Transmigration
Type: Anti-Unit
Rank: EX
Range: 0
Maximum Number of Targets: 1
A Noble Phantasm that can only be invoked in the central chamber of Yamaloka Naraka, his secret weapon and ultimate Noble Phantasm. It compiles all human suffering into a single attack, cursing the opponent's Dharma and Karma such that Yama obtains ultimate Authorities over them. It is born of the same origin as the major Noble Phantasm of the Bodhisattva. As humans expand in population, dive deeper into suffering, and find new ways to hurt each other, the power of this Noble Phantasm increases. In the modern era, even a top-class Servant would find themselves instantly disintegrated by this sort of attack. Even in the case of a survivor of this Noble Phantasm, Yama obtains Authorities that allow him to rip them to shreds due to simply having the right to "remove the negative Karma by any means necessary.
It firmly cements that whatever is within Yama's sightlines are still Sattva, and must continue to cycle through Saṃsāra. Of course, against those who have achieved the ultimate end, true Enlightenment, it can never have power. But against any Sattva, even a god, it is essentially a "Return to square one" power. It goes without saying that Yama would need to drain the Holy Grail dry just to use it once.
I wouldn't count on this one too much. Unlike the Buddha, I don't have nearly enough mana to pull this sort of thing off.