So I'm making a homebrew RPG that will rely on Servant mechanics as they're shown. I haven't made much progress but what I'm posting here is the basic layout for determining the rank of Anti-Unit NPs. I was just wondering whether people would find it an acceptable way to faithfully represent the strength of NPs.
It's a bit long so forgive me for the length.
There are two primary components to determining the level of the ranking of an Anti-Unit Noble Phantasm, the quality of the weapon and the strength of the conceptual effect.
The quality of a Noble Phantasm can never exceed a D-rank. While this seems unusual, it does explain how Lancelot's D-rank weapons are capable of withstanding blows from Excalibur. There are 3 ranks to weapon quality, a normal weapon will have no rank, a high quality weapon will give an E rank and a weapon that has reached legendary status will have a D-rank.
The conceptual effect of a Noble Phantasm will have 3 ranks, each one adding to the previous quality of the weapon. Once again, the strength of the effect will determine how high the rank will be.
To put this in practice, let us consider Gae Dearg and Rule Breaker. Both weapons have a similar effect and are described well so it is easy to determine the basis for their ranking. Rule Breaker is noted for being only as strong as a normal dagger, thus it gets no ranking from quality. It has a powerful conceptual effect however which gives it a 3 rank boost resulting in a final C-rank.
Gae Dearg is an actual weapon that can be used in combat and gets a base D-rank. It's conceptual effect is not as strong as Rule Breaker although it is noted as being very powerful. As such, it gets a 2 rank boost which results in a final result of a B-rank.
Another example would be a comparison between Gae Bolg and Zabaniya. Gae Bolg is an instant kill weapon, albeit limited by the fact that there are ways to counter it. So it would get a base D-rank then 2 extra ranks from its ability which isn't perfect for a total B-rank. Zabaniya also gets 2 bonus ranks from its ability. However, Zabaniya is not a D-rank weapon; while it is still better than a normal weapon as it is a servant's body, it was not able to withstand Excalibur and as such is better described as an E-rank weapon. Adding together total effects, we get the result that Zabaniya is a C-rank Noble Phantasm.
Another observation of power levels can be seen between God Hand and Avalon. While they are technically two different classes of Noble Phantasms, a comparison can still be made. God Hand, a B-rank noble phantasm is capable of nullifying all B-rank and lower attacks. Extrapolating, an A-rank Noble Phantasm blocks all A-rank and lower attacks and Avalon, an EX-rank Noble Phantasm, can block EX-rank attacks. A similar effect can be seen through their regenerative abilities. God Hand can revive the user 12 times while Avalon is capable of constantly bringing back the user from the brink of death an unlimited amount of times assuming they are not completely obliterated. (This is entirely speculation)
Keep in mind that all the above are generalisations. The ranking system works on multiples of ten. As such there may be slight discrepancies on the exact strength of a Noble Phantasm. The actual cause and effects of the weapons in question may also differ. Conceptually, Zabaniya and Gae Bolg are equivalent but against Fragarach, only Gae Bolg would be effective; Zabaniya would, in theory, work against God Hand as it bypasses the armour enhancing effect but Gae Bolg would probably bounce off. Be sure to properly determine exactly how the Noble Phantasm in question works.
Both Gae Bolg and Zabaniya function on the idea that they will kill instantly but only if the target fails a status check. Maria the Ripper also kills instantly but instead requires specific circumstances to be met before it can be enacted. Although I can't find the source, I recall it as a B-rank.
Determining the actual rank of the conceptual effect is somewhat difficult. Gae Bolg and Zabaniya both appear capable of being stopped by the respective B-rank resistance required; Saber with B-rank luck dodged Gae Bolg and Zabaniya could presumably be stopped by B-rank magic resistance although I can't prove that (Lancer with C-rank died to it). As for Gae Dearg and Rule Breaker, they are measured by just how many things they can affect. So if we considered a Noble Phantasm that affected elements, 1-rank would affect 1-2 elements, 2-rank would affect 3-4 and 3-rank would affect all 5. Alternatively it could affect all 5 elements but with differing levels effectiveness.
So when designing Noble Phantasms, there are two things to consider. One is the strength or suitability of the Noble Phantasm as a weapon. The other is the strength of the conceptual effect of the weapon. Combine these and we get the full rank of the weapon. Conceptual effects come in two classes, those that are passive and primarily work on direct contact with the target and those that are activated and affect the target from a distance. The conceptual effect bonus is determined by its effectiveness out of 100% where 100% is a situation where it always works and then rank decreases as effectiveness decreases.
As for why the ranking matters, the strength of a Servant will be determined by a point buy system so the higher the rank, the greater the cost.
I haven't actually gotten to that point yet.
Please criticize.