The Diamond Axe is a magical weapon found in the Kishin Douji Zenki series. It is a golden axe, featuring a dragon-like ornament clutching a green orb.
In the original, Japanese version of the anime and the games, it is known as the "Kongou Kin" (金剛斧), which is pronounced "Kongo Kin". The Kongo or Diamond in its name is related to the Diamond Axe being a source of the holy light, also known as Vajura or Vajra in Buddhism and in the Kishin Douji Zenki series.
Chiaki uses the Bracelet of Protection to call the Diamond Dragon, which then transforms into the Diamond Axe for Zenki. Zenki mainly uses this weapon while 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 Episode 17 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 itself doesn't make a proper appearance until Episode 18, though, when grandmother Saki finds an old pot and uses a candle with it. The pot turns out to be an antique projector and shows a map of Shikigami-chou, as it appeared back in Ozunu Enno's days.
At first, only Saki and Miki Souma are watching the projection, but then Chiaki, wearing pyjamas, comes along and sees it, too. She cites something.
Chiaki: "You must find the one who transcends time and survives three generations. If you do so, the great power shall be yours."
When Saki asks her, where she heard it, she says, that she heard it in a dream. Then Chibi Zenki join them and tells them about, how he already knew about the Diamond Axe's existence.
Chibi Zenki: "Long time ago, I tried to use it and Ozunu said that... ...it wasn't the time yet, and hid it somewhere."
Saki states, that Chiaki might soon possess a weapon of an equal strength to Rudra, which Zenki is going to need to cope with the evils of this world.
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 splits Chimaru into two by making the Diamond Axe appear from his bare hands and swinging it with both in Volume 5. The same volume also has him countering 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.
In Volume 6 it is also used, when Zenki faces Kimon after his transformation into a helicopter. Zenki manages to destroy the helicopter with his axe, but Kimon himself ends up mostly unharmed.
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 Rai 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 Rai Bu
While Den Ei Rai 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 Rai 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 Rai 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 anime dubs and subs commonly mistranslate the Diamond Axe as the Golden Axe. More information on this topic can be found by visiting the the Kongo article, as this issue also affects the Diamond Dragon and Zenki's Diamond Horn.
Some versions of the anime dubs and subs also falsely call the Diamond Axe the Axe of Diva.