Three characters, Amy, Jin, and Sieghart have bitter hate towards the Ascendant God for his evil deeds. He also kidnapped a Starlet by the name of Lilith, and one of her loyalists, Sister Maggie, went to elect Amy as Lilith's successor. The Highlanders created an organization, the Prime Knights, to balance out the evil within Thanatos and elected Sieghart to defeat him.
During the events on Silver Land, a Guardian Ent was freed from corruption by the Grand Chase, who informs them about an even greater "darkness" coming from the lands of Deities and all the inhabitants of Silver Land were completely defenseless from the corruption. The Chase assumed the cause was Kaze'aze, another evil mastermind they were tracking, but realize Thanatos' power is even greater than hers. Eventually, it led down to one man: Victor, who was the mastermind of the Silver Land's corruption, and one of Thanatos' minions. After a long battle, Victor was defeated by Jin, and he asks Thanatos for help. However, Thanatos' voices tells Victor that he was just his tool who outlived his purpose leaves him to die.
After defeating Kaze'aze, the Chase theorize that a greater evil is at work. As they head back to Bermesiah, a voice confirms their suspicious, informing them that a greater evil has already erupted in the Land of the Gods, and asks them for help. Upon reaching the continent of Xenia, the Chase learn that this "Ascendant God" grew in power and broke the "balance" between all the deities. Samsara, one of the Deities in Xenia, devised a plan to overwhelm the powers of Ascendancy by using the other five strengths of the Deities, in which the Chase would go and plead the other Deities' for their essences. As time passed, the other deities were freed from possession and had supported the Chase to defeat Thanatos.
When they reach his lair, Thanatos toys the Grand Chase before ultimately unleashing his true power. However, with the help of the Deity Orbs, Thanatos is overwhelmed. Unfortunately, Ashtaroth, one of Thanatos' loyal guardians, took his Deity Orb and left, where he plans to use it to create the Soul Stone.
When Grand Chase first encounter Thanatos, he takes on the appearance of a young boy with white hair and a mask covering part of his face. Amy calls him "handsome", but its just a form withholding immensive energy.
Two Dark Hierophants and two Sataniel's Warriors assist him in this part of the battle. He has moves similar to Jin and Siren. He has a total of three attacks in battle. His first attack is signaled when he raises his hand, and a burst of fire appears directly on top a player, severely damaging them and launching them high. His second attack, similar to the flower in Jin's White Flower Technique, occurs to damage a player near him, indicated when he lowers his head. The third attack is which Thanatos will raise his arm to one direction, which a heavy sound wave appears in that direction. This wave has infinite vertical and horizontal range which makes it extremely difficult to dodge.
The first form will levitate in one spot and teleport every so often.
Once the Grand Chase defeats Thanatos' first form, he will take on his second form, which looks like a mutated knight.
Two Heras accompany Thanatos during this part of the battle. He uses moves similar to Perseo and Victor alongside with Polaris. He can shoot multiple dark orbs in rapid succession and they travel extremely fast, similar to Perseo and Victor. When he senses a player near him, he can activate the Polaris' Call of Ruin occasionally that scores multiple hits on a player. If a player is hard to attack, even with his orbs, he sends a dark skull flame that slowly, constantly chases the player and if it hits them, it disintegrates into massive damage to the player and others. Sometimes, he will unleash a bind effect over a wide are which drains player's health to restore his own.
The second form will occasionally float around the map, and eventually, out of reach or even blow the map, which makes attacking him impossible.
3rd Form (Final)
After the Grand Chase defeats Thanatos' second form, he will assume his final, true form. He is given a more demonic look while seemingly equipped with bone-like pieces and thorns.
Thanatos is able to create a burst of fire in his hand to massively damage players in front of him, instantly sending them to fatal mode. He can also send down four spikes that can hit players at a distance, leave flames upon his feet, and can send out fast moving tornadoes that removes all the player's mana. He is also capable of using a bind that affects the entire screen and lasts quite long. Often after the bind he will create a large, dark orb that draws players closer to it before the orb explodes, usually killing the player or placing them in fatal mode.
As his life goes down, Gargoyles will start spawning.
Like the first form, the third and final form will remain in one spot to teleport later. However, he will never gain any elevation.
- Thanatos has Armor of himself. It is also said he made Iblis weapons for her before his descent into darkness.
- Upon defeating him a new soundtrack plays ("TrackD").
- He is one of the four bosses that possesses a transformation, but one of the two that has an animation.
- "Thanatos" in Greek means "Death".
- Thanatos is one of two Deities that does not respawn minions prior to their defeat, the other being Starkiln. It is possible that Thanatos was using his minions and then deemed them "useless" similar to how he used and deemed Victor as a worthless being.
- Ashtaroth, upon his ascension to power in Kounat's Collapse, bears many similar traits to Thanatos.
- Both are power-hungry creatures with ambitions of taking over the world.
- Both manipulate and use others to their expense and dispose their minions after they were defeated by the Grand Chase. Thanatos used Victor to conquer Silver Land and left him to die by the Grand Chase's hand; Ashtaroth used Dahlia only to execute her after the Grand Chase defeated her.
- Both possess three different forms as well as god-like power.
- Both were defeated by powerful, legendary objects. Thanatos was defeated by the Deity Orbs; Ashtaroth met his demise by the Klara Libri's clone.
- Like Kaze'aze, Thanatos' last form is more easier and quicker to dispatch than the previous forms due to his massive size.
- There is a pet modeled after Thanatos' 3rd form, named Thanny Boy.
- Iblis suggests Thanatos actually has a 4th form, saying he should be spared if he wanted to change into a "certain" look.
- Prime Knight says I've waited three hundred years to finish this fight, which suggests that Thanatos started his campaign at that time or possibly even before.
- Although Iblis claims that Thanatos was "kind", this is unconfirmed. The description on Thanny Boy states that he "daydreamed as a youngster of seizing control of the universe and wreaking despair in the hearts of gods and men alike" . Thanatos' kindness was most likely a ploy to lower the other Deities' defenses and gain their trust until he began his reign.
- There are certain hints of information that Thanatos was not originally evil, however. For instance, the description on the Iblis Weapons state that he made them for her before his descent into darkness.