The Anime of the Game: They created a 2D anime-style PV featuring Yaobikuni and an all-CGI short created for the game's 1-year anniversary, now used for the game's opening sequence when you first start the game.. A 12-episode anime special in the format of web anime titled Heian Monogatari is going to be aired in Chinese video hosting site Bilibili starting from April 21 2018, with a second season incoming.
There's also the Onmyoji Kindergarten series, A Useless Onmyoji Family, and an unnamed donghua based on the Ooeyama taiji.
Fan Nickname: As a fanon have it, Shiromujō's real name is "Tsukishiro" (actually, it is his skin name, Tsukishiro-no-enishi), and its still sometimes being used by Kuromujō is the key to breaking him out of Wistful Amnesia. Following that line of thinking, Kuromujō's real name is also deemed as "Kurobane" (from skin name Kurobane-no-kizuna).
Onikiri has "Onigiri", "Rice Ball", "Anija" and "Mikazuki Munechika". Calling him by the name the real sword is currently known as, Higekiri, is also not uncommon.
Less commonly, "Noriaki Kakyoin" for Susabi, owing to their shared voice actor, similarly-asymmetrical hair styles, deliberate bishōnen aesthetics, overall personality and attack pattern. Once the similarities sink in it's not difficult to see his 8-shot Scourge: Moon as a 20-meter-radius Emerald Splash, especially once it explodes and showers everything else in sight with projectiles...
Yorimitsu is known as "Yoripotsu" in China, for becoming the official scapegoat of the game, so much so it became a meme.
"Aqua" for Enmusubi-no-Kami. Both are voiced by Sora Amamiya, and are Physical Godesses. Enmusubi-no-Kami is noted to have a fairly awkward kit that's so heavily chance-based that it works against itself, hence the comparisons with a certain "useless" goddess.
Despite, or possible because of it being one of the most popular manga ever released in the west, the 5th event, for Bleach, also seems to not be happening either. Incidentally, Bleach and Nura are both distributed by Viz Media, thus making future crossovers with other intellectual properties under their license unlikely.
The Other Darrin: In the musical adaptations. All characters that appeared in the first musical (Seimei, Ibaraki-dōji and Shuten-dōji) are recast in the second one.
Mariya Ise replaces Mitsuki Saiga as SP Onikiri after the latter got involved in politics on Twitter. This marks the first time a playable character had been recast, their original actor differences from the "main" version notwithstanding.
Tamamo-no-Mae is now voiced by Chifuyu and his fourth skin in SP form is voiced by Shin-ichiro Miki
Notoriously, the Gintama fandom has a field day with this – the Underworld is populated by Gintama voices. Ask any one of them and they'll bring up the ever-hilarious Kuromujō and Shiromujō, who in Gintama have a close Bash Brothers and Vitriolic Best Buds relationship as Shinsengumi members Hijikata and Okita. Other than that, their apprentices Kurodōji and Shirodōji are also pointed out to be Gintoki and Kintoki (who are enemies), and they're commanded by Hangan, who is Katsura, the number-1 enemy of the Shinsengumi.
Some voice actors voice more than one roles in this game, and having two or more characters voiced by the same seiyū fight each other results in this. Having the same character on both sides results in literal Talking to Himself.
Menreiki who literally talks to herself in the story when Seimei temporarily splits her Jekyll & Hyde personalities.
Trolling Creator: If anything, NetEase also got on the April Fools' Day 2018 bandwagon with numerous jokes, including promising the release of a Romance Game spin-off. One of them turns out to be serious: they did lower the SSR rate the next day.
Uncredited Role: Noriaki Sugiyama as Kuro Seimei; Seimei's Chinese and English VA/Chinese and English system voice in Kessen!; the PV narrators; the voice talents for Kuzunoha, Tadayoshi, Ame-onna's husband, Yamata-no-orochi's snake form, and various minor characters.
Earlier concepts for Tamamo-no-Mae's playable unit included a Stance System, where he would transform mid-battle through the use of certain skills, and an idea also existed where he could be a purely Control-based unit. Both of these ideas either clashed with the core of his design, or proved too complicated to implement, so the developers went with what we have now. The stance system was latter used for Takiyasha-hime.
Menreiki was supposed to be able to wield all seven of her masks in combat, though the developers scaled that down to three, since as she is right now, Menreiki is already notoriously Difficult, but Awesome, coding for that many more variables in a battle on a single unit would make her far too complex.
The original idea for Higanbana's flower fields was an entire segment of the Sanzu river running across her enemy's side. Naturally, this proved to be a gigantic resource drain, considering the fact that Onmyōji is a mobile game first and foremost, and the river part was scrapped.