Originally Posted by
fumei
I was thinking about this the other day as well.
In IRL myth, and as I'm sure has been referenced in Fate at some point as well, Ama-no-Sakahoko (天逆鉾) was the spear used by Izanagi and Izanami to churn the oceans and create the land that would become Japan. This is the spear that is stuck in Takachiho, which is also referenced in FGO by that one bond CE (Ryoma's?). Via this bond CE, among other things, we know this is also the spear which imprisoned Oryou on the same mountain until Ryoma freed her.
Then Erice introduced the Ame-no-Sakahoko (天遡鉾). The big difference in name here is that 逆 means "reverse/inverse" which gives 天逆鉾 a literal name of like "spear of inversed heaven", while 遡 means "to retrace", so a literal name would be like "spear of retracing heaven". This is actually reflected in Erice's usage of her NP since it's said to "churn spacetime" and then return "all structures, matter, meaning, and fate to primordial chaos", acting as a "reverse flow of entropy from the netherworld". This basically sounds like the exact opposite of what the gods did with their original spear, so despite sharing the same name, I guess the fact that this is a replica lets it have this different function.
The spear new!Ryoma uses in the event is indeed the same Ama-no-Sakahoko, given with the kanji 天逆鉾. This makes sense, since it's the same spear as Rider!Ryoma encounter, however, they kept throwing out entirely new effects for the spear pretty much every time it came up. One time when he uses it he says that being stabbed by this spear results in an "inversion of existence", causing a "divergence from the world" (存在の反転、世界からの乖離). Sounds kinda like how Erice's replica works, so good enough. Then they said that when it fell from heaven and struck Oryou, it "sewed her to the land". This is later shown to also be very literal, but in a more conceptual interpretation I guess you could think of it like if she was really an existence that once belonged to heaven, and it's inversed, then she's now barred from heaven?
In the last chapter today, during the reunion scene, the unnamed god mentions that while the spear pinned Oryou to the land when it fell, there's more to it. It's "the residue of the Age of Gods that fell from the heavens" (天逆鉾は天より降り立った神代の残滓), But even that's not its true purpose.
It was never meant to destroy evil, and it's not even really a spear to begin with (その鉾の本来の力は魔をうち滅ぼすことではない。 いや、そもそも鉾ですらない). It is actually a "token of atonement" for the malformity that was cast out long ago, so that it may return to the heavens as a new god (天より逆落ちたるは、かの異形を 新しき神として天へと迎えんがため、遥か昔に異形として忌み、 天より流した我の贖罪の印). By using the spear, Ryoma will cause Oryou to be "reborn as a new god, or rather, his guardian" (今こそまつろわぬ異形は新しき神、 いや、おまえの守護者として新生せん) and likewise he himself may call himself a god (そして新たなる神の名を名乗るがいい).
There's some similar stuff about how the spear was sent to Earth so that someone could lead mankind closer to gods and whatnot, but this is said by Amanozako imitating the spear so probably best not to put too much weight to it.
Anyway, every appearance has been difference now, and Ryoma's latest appearance with it has like 3 or 4 different effects that all seem really different, so I don't know what to really think about this thing anymore. This is on top of Erice's profile literally mentioning that her spear is a replica of Ame-no-Nuhoko (天沼矛) which has the exact same description IRL as Ama-no-Sakahoko, so does that mean that one's the "original" spear that created the landmasses and all these other ones are just different, weird replicas? Who knows.