The Weapon’s Refrain (Ultimate) was added in patch 4.31 of the Stormblood expansion.
The Weapon’s Refrain (Ultimate) becomes available at level 70 as part of the “Test World of Ruin” quest, which is given by Wandering Minstrel – Kugane (X:11 Y:12).
The raid has a minimum ilevel of 0 to enter and rewards you with:
Allagan Tomestone of Poetics: 100