Nidhogg was added in patch 3.3 of the Heavensward expansion.
Nidhogg becomes available at level 60 as part of the “An End to the Song” quest, which is given by Aymeric – The Churning Mists (X:7, Y:26) (Main Scenario).
The trial has a minimum ilevel of 205 to enter and rewards you with:
Allagan Tomestone of Poetics: 10