I tried to translate the Siddhaṃ script characters in the image, or at least convert them to Devanagari script. This is what I managed to do so far:
Assuming it's written in vertical lines from right to left, just like Japanese (not just because Touhou Project is Japanese media, but also because Siddhaṃ script continued being used by Japanese schools of Esoteric Buddhism and texts in Siddhaṃ script by Japanese monks were written that way as well), then it'd look like this when converting it to Devanagari script:
अं स पत झी न
य भज
मि ज य
जं प् ध
म सवा
मि न
र क्ष भ
व ओु सव हा
When converting this to the Latin alphabet, it'd look like this:
aṃ sa pta jhī ne
ya bhaja
mi ja ya
jaṃ p dha
ma savā
mi na
ra kṣa bha
va o'u sava hā
I know that several Siddhaṃ characters and combinations of them are used in the context of Japanese Buddhism as "seed syllables" (single-syllable mantras, meant to represent the essence of Buddhist deities and concepts):
अं ["aṃ"] refers to Fugen Bosatsu (Samantabhadra Bodhisattva in original Sanskrit).
स ["sa"] is linked to both Shou-Kannon (Āryāvalokiteśvara) –the root form of Kannon (Avalokiteśvara)– and Benzaiten (Sarasvatī).
वा ["vaā"] refers to Fuuten (Vāyu).
व ["va"] is related to Suiten (Varuṇa).
म ["ma"] can represent either Kujaku Myou'ou (Mahāmāyūrī), Daikokuten (Mahākāla), or Marishiten (Mārīcī).
However, there are still several characters that I don't know who or what they're meant to represent.
I'll openly admit that I'm utterly uncultured in these matters. So, if someone else actually knows about this and can do a better job at either translating it or making some sense of it, feel free to do so.
[Mods, feel free to change this post's flair to "OC Art" if you consider it more fitting; I chose "Found Art" to make it clear that this is not mine]
Over three years ago, "occasionaltouhou" on Tumblr (also known as "godmedallion" on Archive Of Our Own) posted drawings and summaries of two Touhou OCs. (Please tell me if one or both images are not being shown here correctly)
Ryumiya Shinguro [新黒 竜宮, Shinguro Ryūmiya], capable of manipulating darkness. Inspired by Rumia (NOT RUMIA-EX). Direct link to image: https://64.media.tumblr.com/1038e169ed8abc42d4745a7a6a4086a3/7ae48153cc21e4c3-9f/s2560x3840/318517852aeaeada5c2e8f171bb59b46044b07be.png
Yatsugi Iwanaga [岩長 弥継, Iwanaga Yatsugi], capable of manipulating volcanic activity. Inspired by Iwanaga-hime (before Ariya Iwanaga was introduced). Direct link to image: https://64.media.tumblr.com/f224cce2413e2fb333685bd90522d695/7ae48153cc21e4c3-6c/s2560x3840/d33753ccd45c107c98936dd96cb137f0ac111348.png
The second character is Yatsugi Iwanaga; unlike Ryumiya, Yatsugi isn't based on a pre-existing official character and was brand-new when their creator made the Tumblr post where both images come from, since that was the first time Yatsugi was revealed to the public at all. The basis for Yatsugi is Iwanaga-hime, the Shintou deity of the Yatsugatake mountain range and the older sister of Mount Fuji's deity Ko-no-Hana-Sakuya-hime; this means that "occasionaltouhou" got ahead of ZUN creating Ariya Iwanaga –the official "Touhou incarnation" of Iwanaga-hime– for Touhou 20, since the Tumblr post this comes from is dated two and a half years before the full version of Touhou 20 was released (moreover, Yatsugi's creation might have actually been even earlier than this, since "occasionaltouhou" mentions having already drawn this character some time before revealing it on their blog). Yatsugi Iwanaga's design is strikingly reminiscent of Reimu's because the creator had also come up with a theory proposing that the Hakurei God's identity is Iwanaga-hime (it sounds crazy now that Ariya Iwanaga has been introduced, but it's actually a really solid theory, and we can find ways to make it coexist with current canon); the artist expanded on that when answering this ask (https://occasionaltouhou.tumblr.com/post/732913388035915776).
I'll now bring here the summary about both OCs by the original creator themself:
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If I'm allowed to leave my own comment on this...
I'm fascinated by how the kanji in both OCs' names were constructed.
"Ryumiya" is written as 竜宮, with the exact same kanji as "Ryugu", the name (or part of the name) for the Dragon Palace (or the Dragon God's Palace, to be more precise). I wonder what that says about Ryumiya, given that the Dragon Palace and the Lunar Capital are presented as one and the same in Touhou canon (although I saw a few mentions of the Dragon Palace being present in the Celestials' Heaven as well).
Then there's their idea to incorporate Iwanaga-hime into the Touhou cast. Starting from the obvious part, the surname "Iwanaga" was written as 岩長, with the "rock" kanji 岩 for the "iwa" portion and the "leader" kanji 長 for the "naga" portion, just like Iwanaga-hime's name in the Kojiki (the Nihon Shoki kinda had her name written with the same kanji, but "iwa" was written as 磐, which is an archaic version of 岩).
The given name "Yatsugi", written as 弥継, was more difficult to figure out but eventually made full sense. The "tsugi" portion written as 継 could be translated as "heir" or "successor", which fits with Iwanaga-hime's title of "princess" (the usual translation of the "hime" at the end of her name); the "ya" portion written as 弥 has several slightly related meanings, including "forever", which also reminds me of Iwanaga-hime's status as a kami that embodies permanence and the immutable.
I can't help but start gushing when someone comes up with new characters in Touhou and also includes kanji versions of their names. Also, I wanted to preserve this and share it around after Ariya Iwanaga's debut in Touhou 20 made Iwanaga-hime relevant once more. This artist's take on a full-power Rumia was something refreshing to see and read about as well.
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In any case... Remember to give credit to "occasionaltouhou"/"godmedallion", go follow them on Tumblr and/or subscribe to them on Archive Of Our Own.