In the dragon language font (both the one featured here and ripped from the game), the special letters are mapped to the numbers 1-9. However, there is a mysterious rune mapped to 0. See below:

1234567890

The last rune is one that is in the original font file but is never used either in the game, guide book, or other official material. Any theories as to what this unused letter was meant to be? Below is my take.

0

Since it is mapped to one of the numbers, I think it's a special letter of some kind. I don't think it is a duplicate for an existing single letter or a supposed "missing" letter C. The rune for "ey" is mapped to C in the original ripped font, so that would seem to rule out this being a letter for C, though we may never know.

The game guide has the following to say about the alphabet:

The Dragon alphabet consists of 34 distinct runic symbols. While most have direct English-letter equivalents, some represent sounds English uses two letters to represent, such as "th" and "ch". The complete alphabet is as follows:

Excluding our mystery rune, the alphabet does consist of 34 runes as the guide states. It also does not list the mystery rune in the rune chart seen below.

We are dealing with an obsolete/unused rune here, so it's possible that Prima Games wasn't aware of its existence, or if they were, intentionally excluded it. They also appear to be working with a different font than ours. The game has two dragon rune fonts - one for the Word Walls seen above, and one used in the menus and books, which is where our font comes from. It's also possible this mystery rune doesn't exist in the font above. I don't know if the former font was ever ripped from the game so I don't have the means to check.

Going back to the quote, I find it odd/interesting that "th" and "ch" are used as examples since the dragon language does not have runes for "th" or "ch". It may be intentional - they are more familiar examples than "ah", "ey", or "ir". It also might just be poor choice of words.

So what we have: a rune that English uses two letters to represent. What such two-letter combinations don't have a known rune in the dragon language? Here's a list on the alphabet page. I'll go over each possibility and arguments for or against:

​AE

Best explains why this rune is obsolete. "Mahfaeraak" is the only word that uses "ae", which is later represented by "ey". It is used in the Song of the Dragonborn, which was one of the first creations in Dovahzul. Bethesda may have made a rune for "AE" at this early time only to later can it in favor of "EY". This doesn't explain why they didn't change the spelling to "Mahfeyraak", though it should be noted that there are no canon words that contain "eyr" until "Deyra", which was invented later on for the Dragonborn DLC. If it can be considered evidence, this would also explain why "EY" and "AE" are mapped next to each other.

AU

I don't have much evidence against this possibility except that I don't have much for it either. "AU" is featured in 3 canon words: Aus, Nau, and Staadnau. "Staadnau" appears in the Song of the Dragonborn as well, so it could be a similar scenario to "AE" where Bethesda made a rune for it early on and decided to discard it.

NG

"NG" appears in 5 words: Hungaar, Krongrah, Lingrah, Viing, Zohungaar. The mystery rune resembles the letter "G" flipped horizontally. That said, very few of the other letters that are related look similar. If this was once "NG", it's possible Bethesda got rid of it to avoid confusion between a word like "Viing", which does have the /Å‹/ pronunciation, and "Krongrah", which may or may not depending on how you want to put "Kron" and "Grah" together.

SH

"Shul" and "Shaan" are the only words containing "SH". Not much evidence for or against. If Bethesda had made a rune for "SH", I'd think they would have made a rune for "TH" as well, but we only have one mystery rune.

TH

A lot of words with "TH", I think this one is very possible. It somewhat explains the guide using "th" as an example. Continuing on the above, perhaps Bethesda did have runes for "TH" and "SH" made or planned. The problem is, every letter key + every number key = 36 possible runes. Take away C, and that leaves 35 possible runes. Rather than have a rune for "TH" without a rune for "SH", Bethesda may have left them on the drawing board to make room for the more important runes.

Perhaps it could be CH? Since there, is no C in Dovahzuul, it would make since for it to have been removed from the game. I don't really have any evidence other than that, but it would be pretty useful to use that instead of T S H when doing a direct, phonetic translation of a name or modern word.

Commenting since there seems to be some recent interest in the mystery.

I am fairly convinced this symbol is a former rune for "ae." Here's concept art for the dragon rune font:

It is almost identical to the one we know today. One notable difference is that the alphabet is 26 letters, 1:1 with English, compared to the final 34-rune alphabet. The concept of diphthong characters seems present but not fully formed yet (note the runes for ah, aa, ei, and ii after the main alphabet without assigned characters). It may be that they knew they wanted to make diphthong characters but didn't know exactly which, so these extra characters are present in the design as placeholders until they figured that out.

The most noteworthy bit of this piece is the character for "C," which is what we currently know to be the character "ey." As we know the dragon language in its final iteration does not have a rune for "C" at all. Mentioned above, the rune for "ey" is mapped to "C" in the font, so this now makes sense - the rune for "ey" was formerly the rune for "C." This entirely rules out the possibility that our mystery character is a missing letter "C."

It's clear here that Bethesda was developing the rune font and the dragon language at the same time. Here is an explanation for how our mystery rune for "ae" was created and then became obsolete:

• The late Adam Adamowicz created the above font concept with initial font mapped.
• Bethesda drafted a list of possible diphthong characters.
• The letter "C" was given the axe.
• Bethesda added the diphthong characters to the font. Dipthongs for aa, ah, ei, and ii are ones they came up with first. This is why they are mapped to the earlier numbers. It's also why they are in the concept piece above.
• Bethesda created our mystery rune for "ae."
• Bethesda decided to make a rune for "ey," and recycled their old rune for "C."
• Someone realized that "ae" and "ey" have the same pronunciation. "ey" became used over "ae."

I believe this best explains why this rune is now obsolete, and that it is "ae." Further supporting this, "ey" (9) and hypothetical "ae" (0) are mapped right next to each other in the font. One may ask why they didn't remove the rune altogether, but neither did they remove "ey" from its original mapping to "C."