Why and how did the Septuagint come into being?

Septuagint is the Greek translation of the Bible. It comes from the Latin word septuaginta, which means 70. It came into being when the Greek King of Egypt Ptolemy II Philadelphus order 72 elders to translate the Torah (the Jewish Bible) from Biblical Hebrew into Koine Greek (which was a certain old dialect of the Greek language), in order for it to be included in the Library of Alexandria.