“when the First Heir rises…”

In Arthurian romance, a number of explanations are given for Arthur's possession of Excalibur. In Robert de Boron's Merlin, the first tale to mention the "sword in the stone", Arthur obtained the British throne by pulling a sword from an anvil sitting atop a stone that appeared in a churchyard on Christmas Eve. In this account, as foretold by Merlin, the act could not be performed except by "the true king," meaning the divinely appointed king or true heir of Uther Pendragon. As Malory related in his most famous English-language version of the Arthurian tales, the 15th-century Le Morte d'Arthur: "Whoso pulleth out this sword of this stone and anvil, is rightwise king born." After many of the gathered nobles try and fail to complete Merlin's challenge, the teenage Arthur (who up to this point had believed himself to be son of Sir Ector, not Uther's son, and went there as Sir Kay's squire) does this feat effortlessly by accident and then repeats it publicly.

The identity of this sword as Excalibur is made explicit in the Prose Merlin, part of the Lancelot-Grail cycle of French romances (the Vulgate Cycle). In the Vulgate Mort Artu, when Arthur is at the brink of death he orders Griflet to throw the sword into the enchanted lake; after two failed attempts (as he felt such a great sword should not be thrown away), Griflet finally complies with the wounded king's request and a hand emerges from the lake to catch it. This tale becomes attached to Bedivere instead of Griflet in Malory and the English tradition. However, in the Post-Vulgate Cycle and consequently Malory, early in his reign Arthur breaks the Sword from the Stone while in combat against King Pellinore, and then is given Excalibur by a Lady of the Lake in exchange for a later boon for her (some time later, she arrives at Arthur's court to demand the head of Balin). Malory records both versions of the legend in his Le Morte d'Arthur, naming both swords as Excalibur. In some tellings, Excalibur's scabbard was also said to have powers of its own, as any wounds received while wearing the scabbard would not bleed at all, thus preventing the death of the wearer. For this reason, Merlin chides Arthur for preferring the sword over the scabbard, saying that the latter was the greater treasure. In the later romance tradition, including Le Morte d'Arthur, the scabbard is stolen from Arthur by his half-sister Morgan le Fay in revenge for the death of her beloved Accolon during the Fake Excalibur plot and thrown into a lake, never to be found again. This act later enables the death of Arthur, deprived of magical protection, many years later in his final battle.

Now, do you see the connections there?

The sword

There are many important swords in tsc such as, Cortana, Heosphoros, Pheosphoros and The Mortal Sword.

Cortana is based on the legendary sword Curtana, attributed to Ogier the Dane and the legendary knight Tristan. The real sword, Curtana, also known as the "Sword of Mercy", is a ceremonial sword used at the coronation of British kings and queens and is one of the Crown Jewels of the United Kingdom. Cortana is owned by Emma Carstairs.

Heosphoros is a family sword of the Morgensterns, Clary was in possession of it until she used it to kill Sebastian and the Heavenly Fire destroyed it. However, both Heosphoros and Pheosphoros (which was owned by Valentine and then Sebastian) still exist in Thule and Janus is in possession of both of them, in fact, Heosphoros was likely never even owned by the Clary Fairchild of that world, nor was it used on Sebastian. Janus took possession of the sword at one point and began to wield it, and then he took the sword and Sebastian's Phaesphoros with him to Ash’s world (aka the “real” world).

The Mortal Sword also known as the Soul-Sword, Maellartach, and Angel Blade, is the second of the Mortal Instruments given by Angel Raziel to Jonathan Shadowhunter. The Soul-Sword is primarily used to compel Nephilim to tell the truth, mostly during trials. Shadowhunters who wish to have their claims tested and proved may submit themselves to "trial by the Sword," during which a suitable judge, often a Silent Brother, sometimes the Consul or Inquisitor, wields the sword and places it in the hands of the deponent, where it adheres and cannot be removed until the judge wills it. Downworlders and mundanes cannot be compelled by the Soul-Sword, thus preventing the Sword from becoming used by the Shadowhunters as a general tool for interrogation.

The Sword can also have a dark purpose, if intended. Originally, the alliance of the Soul-Sword is seraphic, its power drawn from Angel Raziel himself. However, Valentine Morgenstern discovered an ancient spell to reverse the alliance from angelic to demonic, through a process called the Ritual of Infernal Conversion, where the Sword is seethed until red-hot and cooled four times in the blood of Downworld children: a child of Lilith, a child of the moon, a child of the night, and a child of the fey. With its demonic alliance, the Sword can be used to summon demons and allows its bearer to have control over them.

In September 2012, the Sword was used to question Annabel Blackthorn, who subsequently used it to kill Robert Lightwood and Livvy Blackthorn. It was shattered when Emma struck it with Cortana. Emma and Julian later acquired a version of the sword from Thule, claiming that the Iron Sisters had repaired it and used it to force Horace Dearborn to tell the truth about his involvement with the Unseelie Court and his schemes to gain the position of Consul.

I believe that in twp we will see more of Heosphoros and Pheosphoros in use, but I also think we’ll have a new enchanted weapon, one that will rapresent Excalibur. But if the sword is a parallel with Excalibur, then who is Arthur? In my opinion, the owner of this enchanted weapon will be Kit Herondale and let me tell you why. Arthur was a normal teenage boy who wasn’t aware of his lineage until he pulled the sword out of an anvil. We have three main characters in twp, Kit, Ty and Dru. Guess who is the only one who didn’t know he was a shadowhunter and wasn’t aware of his lineage? Kit. Also, the Arthurian legend is Welsh folklore. Which shadowhunter family is of Welsh orings? Yeah, the Herondales.

I assume The Mortal Sword will once again come in hand, except that this time it's a thule version of it. Also, the only other person who has an analogue arc to Kit's is Clary, and that's why I think the both of them will play a big role in the killing of Janus. Especially if you think about when Jace was once killed with the mortal sword by Valentine.

The lake

In a version of the legend where the sword of the stone and Excalibur are two different blades, Arthur breaks the sword of the stone and is given Excalibur by the Lady of the Lake at the enchanted lake.

The correlation here is obvious, the enchanted lake in tsc is Lake Lyn, also known as the mortal mirror of mortal glass. Ingesting the lake's waters can be poisonous to Nephilim, but it has no effect on Downworlders. Faeries have been known to drink from the lake, saying that it gives them true vision; for the Nephilim, the water causes hallucinations and may even drive them to madness. Among the Fair Folk, the lake is known as the Lake of Dreams or Mirror of Dreams.

The Lady of the Lake is a name used by several fairy-like enchantresses in the Matter of Britain, the body of medieval literature and mythology associated with the legend of King Arthur. They play pivotal roles in many stories, including providing Arthur with the sword Excalibur, eliminating Merlin, raising Lancelot after the death of his father, and helping to take the dying Arthur to Avalon. Different sorceresses known as the Lady of the Lake appear concurrently as separate characters in some versions of the legend since at least the Post-Vulgate Cycle and consequently the seminal Le Morte d'Arthur, with the latter describing them as a hierarchical group, while some texts also give this title to either Morgan or her sister.

The Lady resides in an enchanted realm, an otherworld the entry to which is disguised as an illusion of a lake. I think she may be in fact the Seelie Queen. It would make total sense for it to be her because of her correlation with Kit (the only known living descendant of the First Heir of the Seelie and Unseelie Courts), Ash Morgenstern (the son she had with Sebastian, who is half shadowhunter and half faerie) and Janus (whom she has an alliance with).

According to her backstory in the Vulgate Merlin, the Lady of the Lake was a daughter of the knight Dionas (Dyonas) and a niece of the Duke of Burgundy. She was born in Dionas' domain of Briosque in the forest Brocéliande. Which takes us to the next point.

The forest

Brocéliande, earlier known as Brécheliant and Brécilien, is a legendary enchanted forest that had a reputation in the medieval European imagination as a place of magic and mystery. Brocéliande is featured in several medieval texts, mostly related to the Arthurian legend and the characters of Merlin, Morgan le Fay, the Lady of the Lake, and some of the Knights of the Round Table.

Here too the correlation is obvious, the forest in Idris is called Brocelind. In September 2012, a portion, specifically the center, of the Brocelind Forest was blighted with dark magic by the forces of the Unseelie King, making the area a space where Nephilim runes and adamas-weapons would be ineffective. Making it, in fact, an echanted forest, like in the legend.