Sumarbrander

The Sumarbrander, also known as the Sword of Freyr or the Sword of Summer, is the divine sword formerly wielded by the Vanir god Freyr, who is associated with sunlight, sumertime, fair weathers, and sacred kingship.

He is known to be one of the most powerful swords within the Norse cosmology, being the sharpest sword across the nine realms and fully sentient, capable of fighting against hordes of enemies on his own.

It is said that during Ragnarök, the sword will come into the hands of Surtr - the primordial Fire Jötunn, who shall use him to destroy the nine realms and bring forth ruins across the Norse cosmology, destroying the ancient primordial ash tree upholding the cosmology to its roots.

At the request of the sword, the current wielder - Magnus Chase - changed his name into Jack.

History
The sword was created by the Elder Svartálfar as a gift toward the newly-born Freyr. As he grew up fighting in the Æsir-Vanir War, he used this sword to slay countless gods and monsters alike, marking his presence into the mind of the Æsir gods as a god of nature whose destructiveness rival that of a god of war.

The love of Freyr
Following the union between his twin sister Freyja and the Æsir god Odin, due to the later's constant disappearance and "journeys for knowledge", his sister had to often leave to find him across the nine realms. Worried for his sister's state of mind, Freyr decided to sit on the Hliðskjálf to watch over all the nine realms, wanting to find the locations of both his sister and his brother-in-law. However, instead of what he desired, he fell in love at first sight with a Jötunn woman named Gerðr.

Soon, the fair-haired god became obsessed with the images of the woman he loved that he abandoned his works as a god to make constant trip to Vahalla, for the sole purpose of sitting on the Hliðskjálf and see Gerðr once more. Because of his negligence, humanity began to suffer which led to his attendents to find a way to help their master get back on track. Ultimately, they had decided to help him gain Gerðr's hand-in-marriage.

After hearing about their plan, Freyr was delighted and decided to reward the vassal who thought of it, Skírnir, with his prize horse and his legendary sword if he succeeded getting her to marry him. After several tries, which ended with Skírnir threatening Gerðr into meeting Freyr, leading to both of them felt in love and married, the Sumarbrander was granted to the human vassal, per Freyr's promise.

Despite feeling betrayed by Freyr, Sumarbrander continued to serve Skírnir until his death.

I Play with Fire
While staying with Alex Fierro during Magnus‘ visit with Annabeth, he warns her of that Surtr is planning something. When Alex tells him to come with her, he says he cannot due to his fate being to go to Surt during Ragnarök. He brainstorms with Alex about not getting caught by going alone and constantly shapeshifting.

Appearance
In his dormant form, Sumarbrander's a small white runestone emblazoned with a black Fehu rune - the symbol of Freyr on a golden chain. The stone is magnetically attached to its chain, so he could be easily pulled off, and as soon as it is, the stone grows into a sword with a leather-wrapped hilt that feels warm and comfortable to the hand.

The pommel, a simple polished steel oval, helps counterweight the 76.2 cm (30 inches) blade, which is double-edged and rounded at the tip, more for hacking than stabbing. Down the center of the blade, a wide groove is emblazoned with Viking runes that shimmer in a lighter shade of silver, as if they'd been inlaid while the blade was forged.

Personality
Jack is a voluble sword and is very sarcastic. He likes to flirt with other weapons, and is depressed when they don't return his affections. He also holds a deep grudge against Freyr for giving him up for the sake of love.

Riptide
In The Ship of the Dead, on the boat with Percy, Annabeth, Magnus, and Alex, Jack is excited to see Riptide, saying she's famous. He then goes on to flirt with Riptide. He appears to be 'heartbroken' after he is separated from her.

Spear
According to Jack, he was going on a date with a hot spear.

Magnus Chase
Jack is incredibly loyal to Magnus, offering him advice on how best to wield him and encourages the son of Freyr to fight on his own during Battle Practice.

Freyr
Jack resents the God of Summer for giving him up.

Surtr
Jack fears the Lord of Múspellsheimr as being close to him with start Ragnarök.

Abilities
Jack is a sentient blade; he has the ability to speak and understand human language (although, as he says, he can't speak Álf Sign Language, because he has neither hands nor eyes).

He is able to fly his wielder across short distances, however they do pass out soon afterwards.

Jack is also very effective against the Jötnar, and he can fight them on his own. He is also the only thing capable of cutting through Gleipnir. Whenever the holder of the blade retrieves him after an action, they receive the consequences of the action that Jack has performed.

He has the power to enhance his wielder's magical power and core, allowing them to increase to power of their magic. He also has the power to passively increase the fertility of the land around him.

He is also capable of cutting the fabric of reality opening portals to the nine realms of Norse cosmology. Jack also has the power of a paradox material as he is a "weapon that wasn't designed to be a weapon", a sword that is best used if you let go, meaning he is not just the sharpest sword in the Norse cosmology, but also the strongest. It has also been stated that Jack has a horrible singing voice and it's suggested it could be a form of torture. He also dates with hot magical weapons.

Trivia

 * Most of the information about Sumarbrander was taken from the Magnus Chase and the Gods of Asgard.
 * Sumarbrander is Old Norse for "Sumer Sword".
 * After Sumarbrander begins to speak in The Sword of Summer, his voice is described "definitely male" and is then on described using male pronouns.
 * Sumarbrander is destined to become Surtr's possession and kill Freyr during Ragnarök.
 * In The Sword of Summer, when discussing what form should Jack become when not a sword, Magnus Chase suggests a pen. Jack comments that a pen sword is 'the stupidest thing he's ever heard'. This is a nod at Percy Jackson's sword, Riptide, which becomes a pen when "capped".
 * In The Ship of the Dead, when Jack sees Riptide for real, he tells Magnus that a pen sword wasn't a stupid idea like Magnus thought, even though Jack was the one who thought that.
 * In The Ship of the Dead, Jack reveals he met Heracles, indicating the Norse gods might be older or as old as the Olympians.
 * Jack is a paradox, being a "weapon that wasn't meant to be a weapon" and a "sword best used by letting go".
 * In The Hidden Oracle, Apollo mentions having met a smoking hot god in a Stockholm tavern whose sword wouldn't stop talking. The weapon is assumed to be Sumarbrander and the God is most likely Freyr.
 * Curiously, the runes on Jack's blade don't seem to translate to anything in particular.  When read vertically, it roughly reads "aeaeeof."
 * Jack has the entire cast album of Jersey Boys memorized.