The Seat of Sacrifice was added in patch 5.3 of the Shadowbringers expansion.
The Seat of Sacrifice becomes available at level 80 as part of the “Hope’s Confluence” quest, which is given by Crystal Exarch – The Crystarium (X:12, Y:11).
The trial has a minimum ilevel of 465 to enter and rewards you with:
Allagan Tomestone of Poetics: 12