The Lambs of Dalamud doomsday cult has taken over the Tam-Tara Deepcroft, and Miounne and Lewin ask you to clear them out.
How to Unlock
To unlock the Tam-Tara Deepcroft, you must be at least level 16 in a combat class. The Tam-Tara Deepcroft is unlocked during the level 16 Main Scenario Quest Fire in the Gloom.
Destroy the Cultist Orbs
The path will eventually split, but there is nothing of interest to the room to the east, so you can skip it and go south.
Up ahead, you will find a Dalamud Priest and some Gravediggers at a Cultist Orb. When you kill the Dalamud Priest and Gravediggers, a Void Soulcounter will appear. Kill it, and a treasure coffer will appear. Open it and roll for loot. Then a player needs to interact with the Cultist Orb. This will lower the barrier to the next area.
Follow the path. When it splits, you can ignore the room to the east. It just contains a treasure coffer. Go south and you will find a Dalamud Priest and more Gravediggers. Kill them to make a Void Soulcounter appear, and kill it as well. Then interact with the Cultist Orb to lower the next barrier.
Get the Cultist Rosary
As you go along the path ahead, you can optionally go to the southwest room to find another treasure coffer.
Eventually you reach an area that is blocked by a Sealed Barrier. You will need to get the Cultist Rosary to remove it. Go to the room west of the barrier and kill all the mobs there, then interact with the Cultist Rosary to take it.
Break the Sealed Barrier
Go back to the Sealed Barrier and interact with it to remove it. Kill the Gravediggers and Octavel the Unforgiving to make another Void Soulcounter appear. Kill it and a treasure coffer will appear.
Destroy the Final Cultist Orbs
You can now interact with the last two cultist orbs to destroy them. After you do so, there will be a cutscene. After that, go east to find Galvanth the Dominator.
Galvanth the Dominator
Galvanth will cast Water throughout the fight. He may also cast Mind Blast, a circle attack centered on itself that inflicts paralysis. Eventually he will summon an Imp and tether to it, rendering him invulnerable. You have to kill the tethered Imp before you can resume your attacks on Galvanth. Later, he will summon two tethered Imps and other mobs. Kill the other mobs, then the imps. Repeat this strategy until Galvanth is defeated.
Completing the Quest
Defeating Galvanth will complete one of the quest objectives for the Main Scenario Quest Fire in the Gloom.