|Introduced in|| Manga:|
Kishin Douji Zenki Volume 1
Kishin Douji Zenki Episode 21
The Diamond Axe is a magical weapon used by Zenki in his Demon God and Great Demon God forms.
When Zenki uses Rudra, he is badly injured, but the Diamond Axe is said to yield the same strengh without the drawbacks. It is actually a controlled form of Rudra, which otherwise would hurt Zenki as he can't control it normally.
In one episode of the anime, Zenki gets badly wounded because of using Rudra. He returns to his Chibi form, stumbles at Chiaki and falls over, loosing his consciousness, Chiaki gets very worried. She never wants Zenki to use Rudra again, because she is afraid that he might die, so Chiaki and the other try to find a way to make that possible. While Chiaki and her grandmother Saki are reading old books, they stumble across the legend of the Diamond Axe.
The Diamond Axe first appears in a panel of Volume 1 where Chiaki tells her customers about Zenki's legend. It shows Zenki's sepulcher with Zenki holding the axe.
It later makes its first proper entrance when Great Demon God Zenki counters the Inugami Roh's pentagram seal attack by summoning the Diamond Axe and blocking with it.
The pentageram seal attack consists of Roh flinging his surfboard at Zenki, which catches Zenki in space and has him fight against a meteor. Zenki breaks the meteor and exits the seal, returning to his friends and earning a frown from the Inugami Roh, who didn't expect this.
A sepulcher is some kind of tomb or grave. In Zenki's case it looks like some kind of statue depicting him.
In the Games
The Diamond Axe is an item exclusive to three of the five Kishin Douji Zenki games.
While the Diamond Axe seen in Kishin Douji Zenki 2 - Den Ei Rei Bu and Kishin Douji Zenki 3 - Tenchi Meidou is the one from the anime, Kishin Douji Zenki (Game Gear) features the Diamond Axe's simpler design from the manga instead.
Den Ei Rei Bu
While Den Ei Rei Bu has the Diamond Axe first appear after Zenki defeats Hiruda, it is exclusive to that one cutscene and never gets used in the actual game. This leads to odd things like Zenki defeating Karuma by using Rudra instead of the Diamond Axe, despite the fact that Den Ei Rei Bu is actually heavily based on the Karuma ark of the anime. There's also no explanation why the axe can't be use and after it's introduction it's just simply never mentioned again.
While Tenchi Meidou is actually the first and only game in the series that has the Diamond Axe as an actual usable weapon, it can also only be used by Demon God Zenki.
Yet, things get even weirder, as Demon God Zenki is now always able to use the Diamond Axe. This means that eventhough this game is based both on the anime and the manga, Zenki can use the axe from the very beginning of the game, which is as early as Zenki's and Chiaki's first meeting with Miki Souma, where Zenki shouldn't even have the axe, yet.
It is very likely that while Zenki's Diamond Axe has its anime appearance, it uses the same mechanics as in the manga, allowing Zenki to always summon it in his true form as it's the manifested form of Rudra (Thunder), which is equavalent of Zenki's light(ning) element.
If Zenki charges up enough light(ning) in his body, he is able to unleash Rudra or form the Diamond Axe. As Rudra would cause him to gamble his life, but the manifested form, the Diamond Axe comes without drawbacks, Zenki obviously prefers the Diamond Axe.
So why doesn't Zenki always use the axe, if he can just readily summon it? It is very likely that Zenki has this limitation as he has been covered by a multi-layered seal by Ozunu and needs Chiaki to unleash his true powers. This is only possible by Chiaki gaining experience and developing her abilities though and she needs to be recognized by Ozunu by passing his trials. As long as Ozunu doesn't recognize her as his successor, Chiaki can't unleash Zenki which prevents him from using the Diamond Axe and his later forms.
Two Diamond Axes?!
Interestingly the Diamond Axe from the manga looks vastly different from the one seen in the anime and the games.
The Diamond Axe from the manga is still golden, but instead of taking its design from the Diamond Dragon's appearance, it seems more generic and seems to have spikes on its handle. It also has a chain which isn't seen on its manga and ingame counterparts.
Also instead of Chiaki gaining the Diamond Axe for Zenki by proving herself to the Diamond Dragon like in the anime, the Diamond Axe from the manga seems just readily available to Zenki.
In the manga, Great Demon God Zenki just summons the Diamond Axe. The Diamond Dragon is completely absent in the manga, implying that the manga's Diamond Axe just has an enterily different origin alltogether, being likely linked to Zenki's very existence instead.
It has been stated that the Diamond Axe in the manga is actually made from Zenki's Diamond Horn and infused with Rudra, explaining why Demon God Zenki can always summon it while Chibi Zenki can't. The Diamond Horn has been given to Zenki by Ozunu and Chibi Zenki can't use the Diamond Horn as he's in an even more sealed state than his true form. Anyways, the Rudra (Thunder) is the base of Zenki's very being as seen in the first intro of the anime.
While the Diamond Axe appears in two games, they have it appear with its design from the anime instead of the one from the manga. While this makes sense for Den Ei Rei Bu, which is based on the anime's Karuma Ark, it doesn't make sense for Tenchi Meidou which is heavily based on the series' manga.
The Diamond Dragon (Episode 26)
Zenki and the Diamond Dragon/with the Diamond Axe
Den Ei Rei Bu
After defeating Hiruda
Kishin Douji Zenki (Game Gear)
Kongo Kin, Kongo Ryu and Kongo Kaku
The common mistranslation of Kongo as golden likely stems from the different ways it can be translated and the now, more then ever, obvious fact, that the translators likely took the Chinese versions of the anime and the manga as a base.
You see. When translating Kongo from Chinese to English, it becomes golden just (金剛), referencing to an object being covered in gold or made of gold. It really depends on the context in that case.
Though, this doesn't convey the original Japanese meaning.
If the reader has seen our most recent translation work regarding Tenchi Meidou, they will notice, that Ozunu Enno uses heavily Buddhism related terms when talking and the series in general often relates to things like the Vajra or Rudra. These terms are also often used together, so Rudra becomes Light/Lightning (the reference is less obvious when translating it to Thunder), especially when used alongside Vajra, it becomes something akin to holy light.
So, let's go back to the Kongo with that knowledge. Now there are three common instances of this Kongo, mainly the Diamond Axe (金剛斧, which transliterates as "Kongo Kin"), the Diamond Dragon (金剛竜, which transliterates as "Kongo Ryu") and Zenki's Diamond Horn (金剛角 or こんごうかく, which transliterates to "Kongo Tsuno" or "Kongo Kaku" - Zenki often uses the latter one, which is the child spelling of it).
You remember, how these commonly get mistranslated as the "Golden Axe", the "Golden Dragon", or the "Golden Horn". You already see what happened there, right?
So, the Japanese Kongo can be translated into English as diamond or vajra. The former is being used for the physical forms (aka things you can touch), while the latter is used for holy spells or special moves. (for example, Zenki's well known "Vajura" attack)
I hope, this explains a few things and thanks for reading. -- Zenki on 15:50, July 24, 2017
Interestingly, the Diamond Dragon's horns bear a great resemblance to Demon God Zenki's tiara sans the third eye and middle horn.