To put it simply – I doubt it.
For those of you who have never heard of this prophecy, it is rumored to have been recorded by St. Pio of Pietrelcina in a letter to the Commission of Heroldsbach which was supposedly appointed by the Vatican.
The letter contains documentation of several prophecies allegedly given by Our Lord to Padre Pio in 1949 and 1950 that seem to be referring to three days of darkness that will precede the end of time.
There is much to doubt about the veracity of this so-called letter. First of all, I could find no information about the so-called Commission of Heroldsbach. The only facts I could ascertain about the place is that it is a Bavarian town that was declared an official pilgrimage site by the Bishop of Bamberg in 1998 after reported apparitions of Mary and the Baby Jesus occurred there to four children beginning in 1949. However, according to the University of Dayton’s Marian Library, a negative decision was ultimately reached about these apparitions.
Second of all, there is a glaring lack of reputable sources for this letter, a problem addressed by apologist Jimmy Akin in this article.
I concur with Mr. Akin. This letter is probably not authentic.