[I] [B]the ninth compositiong will kill the composor?! [/B

I'm assuming you mean ninth symphony, not ninth composition. Even then, the myth is a little exaggerated. Dvorak and Vaughan Williams are the only major composers I can think of offhand for whom the myth actually works. (And only in Vaughan Williams's case did death come shortly after completing the Ninth.) Of the other composers usually cited in this respect: Bruckner has three symphonies in addition to the numbered nine (one of them is the "Student Symphony," another is "Symphony No. 0," and another is simply the "Symphony in Bb." Mahler, as you alluded to, has an unofficial symphony in "Das Lied von der Erde" as well as an uncompleted Tenth, and is suspected of having written a few symphonies that were later destroyed. It is true that Beethoven and Schubert were unable to complete tenth symphonies, but both left behind fragments of them. In Schubert's case, there were a good four or five uncompleted symphonies even before he got to the "official" Ninth in C-Major. (And if the uncompleted ones don't count, then he doesn't even have nine symphonies, since the "Eighth" is incomplete too.) Am I leaving anyone out?

It is just a superstition which affected a handful of composers of symphonies. There are plenty of people who ignored it, and suffered nothing. Before Beethoven, many, many composers not only composed more than nine symphonies without ill effect, they composed heaps of them. Mozart composed 41 symponies. Haydn composed more than 100 (104 is the official number, but revisions and compositions which are not catalogued in the "canon" raise the total to 109).

After Beethoven, a signal example is Shostakovich, who wrote 15. Alan Hovaness wrote at least 15, and i believe more.

It was just a superstition, briefly held by those who saw Beethoven as some sort of compositional god.

Quite a lot more... 67 of 'em. 30 of them were written after the age of 60.

Thanks, Boss--i'm not a particular devotee of Hovaness, and only knew that i'd heard his 15th . . .