The term dovahzul is actually a term we made to describe the dragon language in the dragon tongue. The creators of the Elder Scrolls V: Skyrim, also created the game. I have talked to Bethesda about a rune that was missing from the alphabet, but they said that the person who actually made it passed away, so I assume that the creator of it origionally is lost.

Why did he/she died?

No one said and I felt it rude to ask.

Oh, sorry....

There are many speculations about the 35th rune and people regularly scour the game for it, then ask Bethesda. But yes, we will never know