Unknown writing

The  is the unofficial name given to one of the two known writings existent in the universe of the Attack on Titan manga, along with the Walls Writing. It is a mysterious writing that very few people are known to be able to read, including Ymir and Eren Yeager (the latter only in Titan form); and possibly also Ilse Langnar and Levi Ackerman.

Description
The Titan Writing is very similar to the Walls Writing, although it is known to be a different language since none of the Survey Corps members (including Reiner, a warrior from the Hometown) present at Utgard Castle were able to understand it; with the only exception of Ymir. The reason why she can read the writing and whether this is a common trait for the Subjects of Ymir or it is an Ymir-exclusive ability is still unknown. Eren Yeager was also able to write a piece of Titan Writing while in Titan form during Hange Zoë's experiments, but after returning to human form and regaining consciousness he did not remember the incident. Ilse Langnar's notebook appears to be written in Titan Writing, and Levi Ackerman was able to read it when he found the book.

Appearances
The existence of the Titan Writing is revealed when a small group of Survey Corps members (including graduates of the 104th Training Corps) take shelter at Utgard Castle and find several mysterious crates left recently by unknown occupants. They discover that the crates are full of supplies labeled in a strange language they can not understand. However, Ymir inadvertently reveals to Reiner that she is able to read the writing when she reads the label of a can of herring.

Later on, when Hange Zoë is doing experiments to Eren Yeager in Titan form, Eren suddenly appears to be in pain and writes in the ground in Walls Writing "father" and "had me", along with another piece of text Hange describes as so jumbled that she could not read it. The remaining piece of text is actually Titan Writing, meaning "ate"; thus completing what Eren wanted to say: "father had me ate [him]", revealing Grisha Yeager's fate and the source of Eren's Titan Shifting and Coordinate powers. However, upon returning to human form and regaining consciousness, Eren can not remember anything about it, and in consequence, what he wanted to say remains unknown until Rod and Historia Reiss trigger his lost memories about his father's death some days later.

Another possible case of Titan Writing is found in Ilse Langnar's notebook. As she is running trying to reach Wall Rose after the failed 34th expedition beyond the Walls, she writes on her notebook a journal of her experiences, and the text matches the Titan Writing. Ilse is killed by a Titan shortly after, and one year later, Levi Ackerman finds her notebook and seems to be able to read its contents, as he correctly says that the notebook belongs to Ilse and that this is what she died for.

Trivia

 * Like the Walls Writing, the Titan Writing is simple Japanese katakana turned upside down and messily written to make its reading difficult and give the impression that it is a different language. However, unlike the Walls Writing which is read left to right (like regular Japanese), the Titan Writing is read right to left.
 * It is possible that the "Titan Writing" in Ilse's notebook is an inconsitency rather than true Titan Writing. Neither Ilse nor Levi have ever shown any hints that they could be in possession of any special knowledge besides what it is commonly known by all inhabitants of the Walls; with Ilse repeatedly questioning the Titan about its origins and motives and unable to recognize the meaning of "Lady Ymir" and "Subjects of Ymir"; and Levi being ignorant about his own heritage and of the existence of Titan Shifters before the Battle of Trost District. Levi was also present during Hange's experiments with Eren and yet he did not translate the piece of Titan Writing Eren wrote; despite the fact that Hange should be aware of his knowledge as she was with him when Levi found Ilse's notebook, and the notebook was presumably analyzed later (although non-canon, the OVA episode does not make any mention of Titan Writing in the notebook either, and depicts Hange reading it by herself). Ilse's Notebook was released more than a year before the Titan Writing was officially introduced, and unlike the other two cases, the text in the notebook does not reveal anything new nor plays any relevant role, as it is just a transcription of Ilse's inner thoughts seen in the same panel. It is known that Isayama lets his assistants take care of trivial instances of Walls Writing, and back then they had little to no supervision, with some of them adding their personal URLs or sex jokes; so all of this suggests that this case may have been intended to be simple Walls Writing.