Full team (healers, shooters, tankers)
You will appear in the realm of Thanatos, proceed with caution and enter this fortress:
The forces of Thanatos the Eternal Soldiers will try to kill you and your team, defeat them to proceed down.
Underground, you will face the Eternal Guardians, defeat them to continue freely.
After you have defeated the Eternal Guardiands, collect the items from each chest, one chest is to the west, the other is to the south and the last one is to the east.
Once you have all 3 items, go back to the lever in the west side of the labyrinth, and pull it. You will receive an item that will let you proceed to the next room.
Go back to the beginning of the quest, and if you have 'The Sphere of the Gods', pull the lever.
You will be teleported to the throne room of the Heart of Thanatos:
Try to block it so it cannot retarget the healers/shooters.
After you have defeated Heart of Thanatos, an NPC 'Spirit of Thanatos' will appear. Greet the spirit and then agree to destroy his soul and accept the Hero's Task.
After saying yes, a teleport leading to the final room will appear north of the NPC, shortly the NPC will disappear. After a while the teleport will disappear too so be quick!
Prepare your team for the final battle:
After defeating the final boss, enter the TP and choose only 1 of the 2 rewards. Also as reward you will get a free special mount "Magma Crawler" as seen in this picture below.
Good luck and happy gaming!