Nerathor

This magical black-and-gray dragon resides in a deep, dark cave on Siren Isle. To start the fight with this creature, you need to trigger the "The Drowned Lair" event using the Siren Isle Command Map, or wait for someone else to do so. The entrance to the cave where this monster is located can be found at the coordinates: /way 32.27, 64.87. Once the passage to the cave is unlocked, descend into the depths. This opponent can be found at the end of the cave — engage it in battle.

Respawn:

10–15 minutes (verified in patch 11.0.7). However, you won’t be able to start the fight until the cave entrance is unlocked as part of the "The Drowned Lair" event.