Theres barely any info on them, it's all scattered around, so I've collected it.
Ive consulted several experts on Arcaneology. I have also consulted the co creator of Arcane Adventures, techlevel80, the person who created most of the lore. Azlan, the lead developer of Arcane Reborn was also consulted.
The Eleven Unknowns are a group of astranomically powerful individuals, with the weakest being miles stronger than Arthur the Cursebeard. They tend to stay away from each other as if they were to fight, the entire world and parts of the solar system could be damaged. The Unknowns are ranked randomly, their rank does not correspond to their strength. This post contains major spoilers. I have seperated this post into two pieces, the old unknowns, and the new unknowns. The old unknowns are MUCH WEAKER THAN CURSEBEARD. The old unknowns are on a level similar to Theos and Valencia. The new unknowns are MILES STRONGER THAN CURSEBEARD. Information on the unknowns is very sparse and some of it is contradictory, which is why I have decided to separate them into two categories.
Old Unknowns
Zeus, Allfather of the Cosmos, 9th Unknown
Zeus is the 9th unknown, the fallen king of gods. He was overthrown by Prometheus after absorbing huge amounts of Aurem. He is the original owner of the Crown of Kings. Prometheus manages to kill him after absorbing huge amounts of Aurem. Prometheus then takes the throne of Olympus and the Crown of Kings. Durza somehow manages to find Zeus's body and uses a revived Zeus to kill Cursebeard. It is assumed that Zeus is an unbelievably powerful lightning wielder as he is able to fight against Cursebeard, albeit a mentally exhausted and traumatised one.
Hades, Ruler of the Underworld
Hades was the one pulling the strings throughout most of the Arcane story. He controlled Durza to test the limits of dark magic. He is one of the 3 major gods alongside Zeus and Poseidon and his domain is the Underworld. When the player finds out about Hades's manipulation of Durza, they find and defeat him after learning a technique to destroy curses from him. Hades will eventually become an encounter in the Seventh Sea.
Poseidon, Ruler of the Seas, 9th Unknown
Poseidon was one of the three Major Gods alongside Zeus and Hades. He is presumably the creator of the beings that lived in the Sunken Ruins. Poseidon created Poseidon magic, a movement based magic and is a commander of the One Shot Pirates. Poseidon gains the status of 9th unknown presumably after both Zeus and Prometheus die. Poseidon dies in battle in the Seventh Sea against Durza's army during the Cursebeard War. He is presumably revived and defeated once more.
Prometheus, The Gift Giver, 9th Unknown
Prometheus is the god that gave humanity the gift of magic. He appeared as an 'old monk' that somehow managed to discover magic. His teachings spread throughout the world like wildfire. Prometheus had humanity burn Aurem, a valuable gem found within the earth, in exchange for magical power. Prometheus gained huge amounts of power from humanity burning Aurem. He used this power to ursurp Zeus, the 9th Unknown and kill him. Due to this, he gains the status of 9th unknown. He became the King of Olympus and took the Crown of Kings from the now dead Zeus. Eventually, an extremely powerhungry human named Arthur raids hundreds of towns and cities to burn their aurem. This gives him collosal amounts of power, which was an oversight of Prometheus. This power eventually surpassed Prometheus himself. After attaining so much power, he and his crew raided Olympus, many gods were presumed to be killed during this event. Arthur threatened to kill Prometheus as he knew Prometheus was holding back the true power of magic from humanity. He forced Prometheus to create the Sea Curses, four of which are saved for him. Arthur then absorbs the Blaze, Ice, Tide and Energy Curses, making him the first Curse and Supercurse user and promptly kills Prometheus. Prometheus knew that he would be killed, and created 5 Grand Fire curses in hopes that they would one day be used to destroy Arthur, now known as the Cursebeard. Prometheus laid desd for a long time, till revived by Durza in the third Sea. The player and Theos fight and defeat Prometheus with Theos dying during the battle.
New Unknowns
Chaos, Primal Force, 1st Unknown
Chaos is not exactly a 'new' unknown as they are the oldest being in the entire Arcane universe. They are the first unknown and also the strongest. Next to nothing is known about Chaos except for them being the origin of everything and the first primordial god. They are the strongest character in the Arcane universe.
Torren, 7th Unknown
Torren is the current user of the Darkflame Curse, one of the 5 Grand Fire Curses made by Prometheus in hopes they would one day be used to destroy Cursebeard. Torren's ability is to grow strongest through battle, this scales infinitely. It is rumoured that Torren killed Freedrock several times over. Supposedly he is trapped somewhere under the sea in the Old Sea unable to escape. Once again, not much is known about Torren other than he is planned to be added in the game as a postgame boss.
Freedrock, Wise Warrior, 11th Unknown
Freedrock is the teacher of the Radius Fist fighting style and the wielder of the Lazarus Curse. Freedrock can be found on a wilderness frost isle in the 1st Sea and a wildnerness island in the 3rd sea, where he teaches Radius Fist. The Lazarus Curse allows Freedrock to come back to life after death, after coming back to life, he also becomes twice as strong. Each time he dies, the chance of him being revived becomes slightly lower. Freedrock has died approximately 500 times (The true number may be much much more), which has multiplied his power to ridiculous levels. It is believed that Torren, another unknown, has killed Freedrock many times. In the Arcane Adventures webtoon, Freedrock was shown destroying an entire island with only his will.
Peacekeeper (The Player), 9th Unknown
In the future, the Peacekeeper eventually manages to defeat Durza, and the one controlling Durza, Hades, ruler of the Underworld. After defeating Hades, the player takes the absorption curse and goes on a quest to absorb all curses. The player wants to do this so that they can destroy the curses by using a technique learnt from Hades. Throughout their journey, the player becomes incredibly powerful and replaces the now dead 9th unknown, Poseidon. More info on the player's adventure can be found in the Lore page. The player eventually forms the continent of Magius, more info can also be found on the World of Magic wiki.
Averill
Averill is alive thousands of years later and is a silent ally of the Peacekeeper. In the fourth sea, the Peacekeeper helps Averill steal the Light Curse from an extremely well guarded fortress. This fortress is owned by the pirate crew that wiped out the Patch Pirates, Averill's original pirate crew before leading the Shining Pirates. The Light Curse allows Averill to become even more powerful and also extend his lifespan. He is one of the few curse wielders left on the planet after the Peacekeeper's Curse purge. Averill eventually goes on to sail across the world and eventually ends up where he first started. This leads to the realisation that the world is round. In the game, Averill has a bounty of 50,000,000 Lamina.
Trigno, The Volcano
Trigno is alive thousands of years later and is a silent ally of the Peacekeeper. He extends his life through the Magma Curse and is one of the few curse users left after the Peacekeeper's Curse purge. He is still presumably the captain of the Rumbling Caldera. Trigno was originally part of the One Shot Pirates, Cursebeard's crew. He was part of the crew during it's extremely early days. When Arthur was forming his Supercurse cube, Trigno stole the 5th curse Arthur wanted, the magma curse. This caused him to get kicked off the crew. It was believed he failed in absorbing the curse, but it turned out he did absorb it succesfully. He was left stranded in the middle of the ocean but managed to use his curse to form a path of obsidian which allowed him to survive. In the game, he has a bounty of 400,000,000 Lamina.
Morock Creed, the Hound of God, 5th Unknown
Morock, the Hound of God, wielder of the Promethean Fire Curse, one of the 5 Grand Fire Curses, was one of the 11 unknowns. Morock was an extremely powerful pirate with a bounty of 5,000,000,000 Lamina. He wielded Morock's Sword, an extremely powerful exalted magic weapon. He is the father of Angel Creed, a character who plays a promenant role in the Third Sea's storyline. He was defeated in battle by Valencia, Grand Admiral of the A.G, mother of Sage, and the user of the shockwave curse. His ship, the Bermuda Locket, was sunk and he died with it. Morock's daughter, Angel tries to revive him in hopes that his curse would allow him to vanquish all evil in the world. The player manages to partially interrupt the spell which makes the resurrection only work partially. Morock's soul is freed from the Bermuda Trench and he reveals that if his body were revived, he would destroy the World from rage. This is because he died in battle and his final moments were filled with anger, rage and despair. Morock manages to transfer pieces of the Promethean Fire Curse to Angel, thus granting her Promethean Fire Magic. The rest of the Curse was ejected into Orbit. Morock's corpse also holds the item Morock's Blessing.
Possible Unknowns
These Unknowns are not confirmed but several people believe them to have been Unknowns.
Kronos 9th Unknown
Kronos was the father of Zeus and leader of the Titans. It is unknown if he shared similarities to his Greek counterpart. It is likely Zeus overthrew him and took his title of 9th Unknown.
Uranus, 9th Unknown
Uranus was the father of Kronos and primordial of the sky. It is unknown if he shared many similarities with his Greek counterpart. It is likely Kronos overthrew him and took his title of 9th unknown.
Other Primordials
The other primordials are the beings that were spawned from Chaos. This may include Creation, Destruction, Magic etc (These entities have all been theorised as unknowns, although there existence is unconfirmed). Many of them are likely still alive but dormant.
Olympian Council
The Olympian Council was not a single person, but a group whos combined power allowed them to be considered as an unknown. It is likely this group was made of Zeus's siblings. This group probably collapsed after Cursebeard's raid on Olympus.