That's a fairy tale. The extant Hebrew Scriptures were translated by Jewish rabbis into Greek over a period of two centuries beginning in the 3rd century BC. It was completed before Christ (and, hence, before there were any Christians).
Not a fairy tale. Documented fact.
From Rabbi Singer:
...the Septuagint in our hands today is not a Jewish document, but rather a Christian one. The original Septuagint, created 2,200 years ago by 72 Jewish translators, was a Greek translation of the Five Books of Moses alone. It therefore did not contain prophetic Books of the Bible such as Isaiah, which you asserted that Matthew quoted from. The Septuagint as we have it today, which includes the Prophets and Writings as well, is a product of the church, not the Jewish people. In fact, the Septuagint remains the official Old Testament of the Greek Orthodox Church, and the manuscripts that consist of our Septuagint today date to the third century C.E. The fact that additional books known as the Apocrypha, which are uniquely sacred to the Roman Catholic and Orthodox Church, are found in the Septuagint should raise a red flag to those inquiring into the Jewishness of the Septuagint.
Christians such as Origin and Lucian (third and fourth century C.E.) had an enormous impact on creating and shaping the Septuagint that missionaries use to advance their untenable arguments against Judaism. In essence, the present Septuagint is largely a post-second century Christian translation of the Bible, used zealously by the church throughout the centuries as an indispensable apologetic instrument to defend and sustain Christological alterations of the Jewish scriptures.
The fact that the original Septuagint translated by rabbis more than 22 centuries ago was only of the Pentateuch and not of prophetic books of the Bible such as Isaiah is confirmed by countless sources including the ancient Letter of Aristeas, which is the earliest attestation to the existence of the Septuagint. The Talmud also states this explicitly in Tractate Megillah (9a), and Josephus as well affirms that the Septuagint was a translation only of the Law of Moses in his preface to Antiquities of the Jews.1 Moreover, Jerome, a church father and Bible translator who could hardly be construed as friendly to Judaism, affirms Josephus' statement regarding the authorship of the Septuagint in his preface to The Book of Hebrew Questions.2 Likewise, the Anchor Bible Dictionary reports precisely this point in the opening sentence of its article on the Septuagint which states, "The word 'Septuagint,' (from Lat septuaginta = 70; hence the abbreviation LXX) derives from a story that 72 elders translated the Pentateuch into Greek; the term therefore applied originally only to those five books."3
In fact, Dr. F.F. Bruce, the preeminent professor of Biblical exegesis, keenly points out that, strictly speaking, the Septuagint deals only with the Pentateuch and not the whole Old Testament. Bruce writes, "The Jews might have gone on at a later time to authorize a standard text of the rest of the Septuagint, but . . . lost interest in the Septuagint altogether. With but few exceptions, every manuscript of the Septuagint which has come down to our day was copied and preserved in Christian, not Jewish, circles."4 This is important to note because the manuscripts which consist of our LXX today date to the third century AD. Although there are fragments which pre-date Christianity and some of the Hebrew DSS agree with the LXX, the majority of manuscripts we have of the LXX date well into the Christian era. And, not all of these agree.
Footnotes:Josephus, preface to Antiquities of the Jews, section 3. For Josephus' detailed description of events surrounding the original authorship of the Septuagint, see Josephus, Antiquities of the Jews, XII, ii, 1-4. 2 St. Jerome, preface to The Book of Hebrew Questions, Nicene and Post-Nicene Fathers, Volume 6. Pg. 487. Hendrickson. 3The Anchor Bible Dictionary. Excerpt from "Septuagint," New York: Vol. 5, pg. 1093. 4 F.F. Bruce, The Books and the Parchments, p.150.
Septuagint Online, Kalvesmaki:
Philo of Alexandria (fl. 1st c CE) confirms that only the Torah was commissioned to be translated, and some modern scholars have concurred, noting a kind of consistency in the translation style of the Greek Penteteuch. Over the course of the three centuries following Ptolemy's project, however, other books of the Hebrew Scriptures were translated into Greek. It is not altogether clear which book was translated when, and in what locale. It seems that sometimes a Hebrew book was translated more than once, or that a particular Greek translation was revised. In other cases, a work was composed afresh in Greek, yet was included in subsequent collections of the Scriptures. By observing technical terms and translation styles, by comparing the Greek versions to the Dead Sea Scrolls, and by comparing them to Hellenistic literature, scholars are in the process of stitching together an elusive history of the translations that eventually found their way into collections.
So....even if the whole of Tanakh was completed in LXX before Jesus, the Torah portion is the only reliable part. Nobody knows who or when the rest was done by. We do know that the authors of the LXX did not accurately translate the prophets into Hebrew. And it was further tampered with over time.