Graven Deep Dungeon Guide ESO

Zelvraak the Unbreathing is the final boss in Graven Deep. He is the hardest boss in the ESO Lost Depths dungeon DLC and will you require some coordination and situational awareness to prevail. Before starting the boss you will need to defeat a Flesh Colossus and some ghosts. These don't do too much. Just watch out for the Flesh Colossus as he can hit quite hard.

How to defeat Zelvraak the Unbreathing in ESO

Zelvraak has about 8.2 million HP. There are multiple phases to the fight, and quite a few mechanics. These are outlined below:

Ice Pools

The boss will spawn ice AOEs under the feet of players. You should move out of these when you see them spawning.

Tank Cleave

The boss will target whoever has aggro with a long conal cleave attack. This follows the player (generally the tank) around and stretches all the way to the edge of the room, so the tank should stand still to avoid catching group members in the cone.

Shades

At 75% and 25% health, the boss will split into four shades. These will channel an ability that needs to be  interrupted, therefore each players should stand next to a shade.

Sea Orb

This is the most important mechanic - you must keep on top of it to succeed. Zelvraak summons a Sea Orb that floats in the air and gradually descends to ground level. If it reaches the ground it explodes, which will instantly kill all players, therefore you should assign a player to keep an eye on the orb and damage it to stop its descent. When it takes some damage it will float upwards again. Keep doing this whenever you see an orb descending. 

Fear

The boss will sometimes fear the entire party. You cannot break free from this and it lasts for several seconds, which can be very dangerous and send you running into AOEs. Luckily you can avoid the fear by looking away from the boss when it is about to happen. He will say something like "Gaze upon true fear" just before casting this spell.

Sundered Soul

A random player will be targeted with Sundered Soul, where their soul is torn from their body. The afflicted player with have a strong DoT on them, and they need to find and collect a golden ghost to retrieve their soul, before the DoT kills them.

The boss will also gain some extra abilities depending on the class of the player who was sundered, for example a Sorcerer's soul will allow the boss to send out extra shock AOEs, and a templar soul will create Nova AOEs, and so on.

Zelvraak's Realm

At 50% health, Zelvraak  will send everyone into a different version of the arena, where you will gradually take damage. You need to run around and collect the glowing ghosts to heal yourself through this phase. Stay away from the middle of the room, because the boss will drop an AOE there after a few seconds.

There are also smaller ghosts that will slowly glide towards the middle of the room, where the boss is doing a ritual. You should try to destroy as many of these ghosts as you can. The more that reach the center, the stronger the Flesh Colossus add that spawns in the next phase will be. 

Flesh Colossus

When you return from the portal, there will be a flesh colossus add alongside the boss. This hits the tank quite hard, so it should ideally be focused down.

Inferno

Zelvraak can target the entire group with Inferno, sending consecutive projectiles towards you. When this is happening your healer should concentrate on healing through the damage. 

Zelvraak the Unbreathing Hardmode mechanics

On hardmode, Zelvraak will have around 12.3 million health. His attacks generally hit a bit harder here. The boss will periodically shoot fire orbs everyone in the group - they keep firing every second and this will persist for around 20 seconds. The orbs hit fairly hard so the healer will need to keep on top of this damage to keep the team alive. Two players at a time will be targeted by Sundered Soul.

Finally, there will be four Sea Orbs in hardmode, and if any of them reach the ground then the team will immediately die. You may want to assign players in your group to watch over one or two of the orb locations to make them easier to keep on top of.