Orient is an alternate history manga set in the Sengoku Jidai, as mentioned in the first page of the manga, the Point of Divergence is when the Kishin appeared and killed all the famous Sengoku Daimyo.

https://tvtropes.org/pmwiki/pmwiki.php/Main/AlternateHist ory

We don’t know what exactly caused the Kishin to appear in the human world, but I have a theory that they appeared because of Oda Nobunaga’s actions at Enryaku-ji on Mount Hiei. In Japan, north-east is considered an unlucky direction that evil spirits pass through, so it’s called the Kimon (鬼門) which translates to Demon Gate. Some people believe that building temples facing in that direction helps protect important landmarks, Kyoto is one of these landmarks, Enryaku-ji was apparently built on Mount Hiei which is in the Kimon direction from Kyoto in order to guard the capital. Though there are people who say this is not the reason why Enryaku-ji was constructed as it was built 6 years before Kyoto even existed.

Here’s a picture from the link below showing a diagram illustrating how this works.
https://i.imgur.com/0Kfn2qG.gif
https://www.onmarkproductions.com/html/monkey-koushin-p3. html

The Siege of Mount Hiei (September 30, 1571) an event where Oda Nobunaga set Enryaku-ji on fire, killing anyone trying to flee even women and child. Here’s a video on the subject.
https://m.youtube.com/watch?v=nyMrrc8UGCA
>The Massacre at Mount Hiei | Sengoku Jidai Episode 24

So going by the logic that Enryakuji was built as and acted like a barrier for Kyoto against the Kimon, attacking that place is probably what resulted in the appearance of the Black Kishin.

This map shown in chapter 46 presumably showing where the Black Kishin/Demon God first touched down, roughly matches the location of Enryakuji on Google Maps. Earlier in the chapter, we see the Black Kishin appear on top of a mountain with what appears to be Oda Nobunaga (the Oda clan Mon/Emblem is there) watching its descent.

https://i.imgur.com/YYKk9Zy.jpg

https://i.imgur.com/I6a4Wtg.jpg

Also, since Orient is alternate history most of its supporting cast is made up of fictional descendants of real life historical figures. The closest comparison I can think of is the Lupin the Third series by Monkey Punch, where the MC Arsène Lupin III is basically an OC grandson to the original Arsène Lupin created by Maurice Leblanc. Also Goemon Ishikawa XIII is a thirteenth generation descendant of the real life Goemon Ishikawa.

https://tvtropes.org/pmwiki/pmwiki.php/Main/HistoricalCha ractersFictionalRelative

Takeda Shingen >>> Takeda Naotora
Hattori Hanzo >>> Hattori Tsubame and Hattori Tsugumi

Osafune Mitsuru (from the Daitou Mines arc) seems to be a descendant of the swordsmiths from the Bizen Osafune School, Bizen Mitsutada, Bizen Nagamitsu, Bizen Kanemitsu etc. The Bizen Osafune Japanese Sword Museum displays swords made by the aforementioned swordsmiths and has a workshop where people still practice sword crafting.

https://m.youtube.com/watch?v=z9l9dgEEaPw
>Bizen Osafune Japanese Sword

Uesugi Kenshin >>> Uesugi Tatsuomi though probably not related by blood since Kenshin never had any biological children instead choosing to adopt.
Sanada Yukitaka > Sanada Nobutsuna or Sanada Masayuki >>> Sanada Aoshi
Yamamoto Kansuke >>> Yamamoto Shunrai
Shimazu Yoshihisa > Shimazu Yoshihiro >>> Akihiro’s father > Shimazu Akihiro and his siblings.
Amago Tsunehisa > Amago Haruhisa >>> Amako Katsumi and his siblings.
Usami Sadamitsu >>> Usami Kuroko
Naoe Kagetsuna > Naoe Nobutsuna > Naoe Kanetsugu >>> Naoe Kanetatsu
Amakasu Kagemochi or Amakasu Kagetsugu >>> Amakasu Masaki
Kakizaki Kageie >>> Kakizaki Keigo

This is pretty neat since it coincides with one of the themes in Orient that being Legacy, best showcased in chapter 41.