I googled the phrase “mysterious manuscript,” and almost all the results pertained to one and the same text.
The Voynich manuscript is a handwritten book of more than 200 pages. Someone created it in the 15th century. It is written with an unknown alphabet in an unknown language. There are extensive color drawings of people, plants, and fanciful creatures throughout. Many of the pages fold out, as in the illustration below.
Extensive efforts have been made to identify the language used, if any; none has been successful. I have supposed the language might be Georgian, which is unrelated to any of the other languages of Europe and likewise has its own very different alphabets — plural. But this appears not to be so, either. Maybe it is no language.
If it is no language, who would fill more than 200 pages with handwritten gibberish? I suspect the work of a madman.