Sephirot was added in patch 3.2 of the Heavensward expansion.
Sephirot becomes available at level 60 as part of the “When the Bough Wakes” quest, which is given by Unukalhai – Mor Dhona -The Rising Stones (X:6, Y:5).
The trial has a minimum ilevel of 190 to enter and rewards you with:
Allagan Tomestone of Poetics: 10
Sephirot, the Fiend, was a deity once worshipped by a tree-like race native to Meracydia. Brought into being for the sole purpose of repelling the Allagan invasion, Sephirot inflicted ruinous losses upon the imperial army before finally being subdued and imprisoned in the central facility of Azys Lla.
And now, millennia later, this immensely powerful entity threatens to awaken, his vengeful dreams disturbed by the careless ambitions of Archbishop Thordan VII. Topple the first pillar of the Warring Triad, or wheresoever his colossal shadow falls shall be visited by unthinkable catastrophe!