Ghosts of the Deep is Destiny 2: Lightfall’s first dungeon. The dungeon takes place in the New Pacific Arcology, and you may recognize this once-inaccessible area from the Red War’s Utopia mission on Titan. Although it drops you in the forest of the Arcology on Titan, this dungeon isn’t part of Season of the Deep. Bungie requires players to purchase a Dungeon Key — if you have the Annual Pass edition of Lightfall, you’re good to go.
Additional guides:
- Destiny 2 Ghosts of the Deep Loot Table - What Each Encounter Drops
- Destiny 2 Ghosts of the Deep Title Guide — How to Get Ghoul
The recommended Power for Ghosts of the Deep is 1790 on Normal mode, and to enter the dungeon, meet Suraya Hawthorne at the Tower to get the Rise quest. Everyone in the fireteam must have this quest to access the Ghost of the Deep dungeon for the first time.
After the opening encounter and diving puzzle, the new objective says "Reach the Wreckage." This guide will help you navigate the shipwreck and complete the Ecthar boss fight encounter.
Reach the Wreckage for the second encounter
On the sea floor, portals in the rocky walls take you to coral caves filled with Lucent Hive. As you head toward the Lucent Hive shipwreck, you'll rotate through sections of grabbing air bubbles and fighting enemies in caves.
When you reach the shipwreck, jump up the slanted walls while defeating enemies that are in the way. Eventually, a dead end leaves you with the only option of sliding down one of the several tunnels in the wall. Sliding down the tunnel can be lethal, but you can jump onto the waterfall structure on the left or try to get to the bridge on the right side. Then, head up the water canal and dive into the methane cave.
Although enemies never spawn in the sea, you're not alone here. In this one-sided game of tag, the second encounter boss will chase you as you frantically get lost in the cave. However, all you need to do is head to the opposite side of the cave and jump up the air shaft.
Ecthar, The Shield of Savathun encounter
First, defeat the three yellow-bar Blistered Knights. They spawn by the left and right staircases and the platform at the front of the room.
Next, defeat the Keeper of the Deep Ogre on the platform to trigger a Deepsight buff to spawn — which you can find in the center of the room by the soulfire pit. Ecthar chasing you in the room will add a sense of urgency to complete the tasks, and enemies spawn in constantly.
Using the Deepsight buff, look at the hanging chains and plates with Hive symbols on the left and right walls. Memorize all three visible symbols and find them in the methane cave below. You can get to the methane cave from the holes in the ground near the Deepsight buff.
Send one or two fireteam members to complete the Hive symbol puzzle in the cave, while someone clears ads in the room.
Ecthar cave symbol location
For those in the cave, Ecthar will chase you the entire time you are underwater. Search for the matching Hive symbols and activate the Hive rune.
You can activate more than one symbol in the methane cave, but the sparse air bubbles may make it difficult. Note, if you get hit by Ecthar in the cave, your Pressure Resistance takes severe damage.
Also, activating the incorrect rune will kill you. Thankfully, the symbols underwater will be in the spot every time you run this encounter. There's a handy map of all underwater symbol locations on Reddit that can help you.
Ecthar Vestiges room
From the air shaft, head back up to the room. Players in the room should defeat the three Lightbearer Wizards, one for each rune activated. Like the opening encounter, crushing the Hive Ghost starts a Vestige countdown for 80 seconds.
In the room, you’ll find three Thrall-like Hive statues. Imbue each one with a Vestige. One is in the front of the room, while the others are by the side staircases. If your character dies and loses the Vestige, another Wizard will spawn, and you can get it again.
Ecthar damage phase
Once that's done, the boss damage phase will begin. Defeat the yellow-bar Wellkeeper Knight that spawns in the back of the room and step into the green Piercing Light buff — which last for about five seconds — to remove Ecthar’s shield. The boss is still mobile in the damage phase, which makes it difficult to stay in one place. 
Quickly deal damage to the boss and clear ads. When Ecthar becomes immune, repeat the Hive symbol mechanic and imbue Vestiges to get another damage phase. Remember, crush the Hive Ghost when you defeat Ecthar!
Ghosts of the Deep Ecthar boss fight tips
You can use a rocket launcher or grenade launcher to deal damage, but opting for something close-range keeps you on the safe side. Shotguns like Legend of Acrius and the Tractor Cannon debuff work well, and Arbalest can remove Ecthar’s shield in one shot. However, close quarters make swords and machine guns ideal weapons. Additionally, Well of Radiance is good to have since Ecthar does rush up on you.
Final jumping puzzle
Proceed to the exit in the methane cave, and traverse deep into the shipwreck until you reach the final boss arena. When the area name appears as Ghosts of the Deep on your screen, prepare to face yellow-bar Shriekers and enemies while jumping through the obstacle course. 
Finally, defeat the yellow-bar Ogre under the gazebo and advance to the final boss fight. (psst, the second secret chest is above the gazebo!)
