Ecthar, the Shield of Savathun

Ecthar, the Shield of Savathun is a boss in the Ghosts of the Deep dungeon. Its shield only takes damage after the Wellkeeper Knight by the rally banner is killed and its green puddle is stood in. Once the shield is down, Ecthar can be damaged from anywhere.

Killing the enemies in the upper arena summons the Keeper of the Deep, an ogre. Its defeat unlocks a Deepsight node that lights Hive symbols in the underwater section. Matching the three symbols spawns Lucent Lightbearer Wizards, and defeating them yields Vestiges of Light that start the damage phase when banked at the Hive statues.