Tree of Names

The Tree of Names is a tree sacred to the Gau Drashari and important to the Pact of Crown and Throne to which Avalir is beholden. It writes a protective spell against extraplanar threats across the face of Exandria while Avalir travels the leylines, though this purpose is kept secret from the city's wizards out of fear that they may use this for personal gain.

Since the Replenishment following the ascension of the Raven Queen, 119 years prior to the events of Exandria Unlimited: Calamity, it has been protected by a piece of arcane equipment known as the Arboreal Calix.

Description
The Tree of Names is a beautiful blossoming tree with dark wood and dark leaves located in the Meridian Labyrinth of Avalir. It hovers in the center of the Arboreal Calix, a stone chamber created to protect it. It interacts with all other Planes of Existence simultaneously; Loquatius Seelie smells the Feywild when standing near it and surmises that "maybe nothing's very far from here", and Patia Por'co speculates it grows through the leylines of the planes.

The Gau Drashari created the Tree of Names to write the runes of a protection spell against extraplanar threats, specifically by writing "the names of those things beyond which should not come", as it dropped blossoms across the world. It also captures anything moving into Exandria from another Plane of Existence, such as Evandrin Alterra and a model of Avalir when they returned from Celestial Plane.

It seemed to have additional protective properties against some extraplanar entities already on Exandria, as fiends seen in Avalir would vomit blood and blossoms. The Tree is capable of defending itself when it is touched or when its secrets are at risk: it grows quickly around whoever is touching it and can attack those nearby with its branches.

It also is indicated to have a connection to the Ritual of Seeding.

Arboreal Calix
The Arboreal Calix was installed over the Tree of Names to protect it, during the first Replenishment after the Raven Queen ascended, by the Gau Drashari. The Calix is a massive stone chamber around two arcane engines with glowing runes and large-bladed fins that encircle glowing blue amethyst orbs. It has stained glass windows depicting Avalir that are glowing with runes and light from another plane of existence. It is opened by turning a massive wheel.

Since then, "Arboreal Calix" has supplanted "Tree of Names" as the term people use.

While Laerryn is unaware of what the machinery does, understanding it as only an ancillary engine of the city's Ley-Rudder, she is aware that over half of the raw magic energy required as a tithe per the Pact of Crown and Throne (twenty-five percent of what Avalir collected) goes solely to the Arboreal Calix. After Laerryn successfully transported the model of Avalir to another plane, she found that the Arboreal Calix drew noticeably more magic.

Background
The Tree of Names was created by the druids of the Gau Drashari to sit in Avalir when it was first lifted off of Mount Ygora 292 years before the events of Exandria Unlimited: Calamity. Because Imyr Por'co and his fellow wizards intended to fly along the leylines, the Gau Drashari felt that this would be good opportunity to create a tree that would write a protection spell across Exandria and prevent the arrival of extraplanar threats. The leader of the Gau Drashari did not trust the wizards would protect the tree if they knew its true purpose and would instead use its power for personal gain, so the Gau Drashari refused to tell the people of Avalir what the tree was truly doing.

The Pact of Crown and Throne was then arranged, and the Tree of Names became a key artifact of the city, one that is "incredibly, profoundly important to the significance and safety of Avalir". The Eyes of Avalir were tasked with forever protecting it. Members of the Eyes were instructed that mere mention of the Tree of Names was an indication that something terrible was possibly about to happen, continuing even after the term "Tree of Names" became uncommon.

During the first Replenishment after the Raven Queen ascended to godhood 119 years before Exandria Unlimited: Calamity, the Gau Drashari built the Arboreal Calix around the Tree of Names to protect it and updated the Pact. It is not necessarily clear what changes were made to the Pact, but after the construction of the Arboreal Calix, the mages of Avalir were to tithe twenty-five percent of the energy they collected in their travels to the Gau Drashari and half of that portion was to go to the Calix. "Aboreal Calix" then became the more common term from then on.

Exandria Unlimited: Calamity
While investigating Vespin Chloras in, Cerrit Agrupnin, Senior Sightwarden of the Eyes of Avalir, discovered an old recording in the archives of the Herald's Tome where Vespin mentioned the Tree of Names to Loras as something that Loras did not know about. Cerrit remembered that the Eyes of Avalir are told about the significance of the tree and relayed this to the rest of the Ring of Brass. Zerxus Ilerez mentioned that, in his earlier dream, there had been a tree held in the palm of a Betrayer God and that the blossoms of the tree fell on Evandrin. He felt this related to Evandrin's disappearance. Simultaneously at the Hall of Prophecy, Nydas Okiro discovered that, two weeks prior, an oracle who had been going mad prophesied that the Tree of Names would soon "no longer scribe the name of our deliverance", resulting in the fall of Avalir, the fall of all civilizations, and the gods leaving Exandria. This prophecy was deemed false by the Guildmaster of the Guild of Divination and hidden, even after other oracles began to go mad as well.

When the city's batteries were attacked later that night, in, the Arboreal Calix requested to requisition thirty percent of the city's magical energy. Architect Arcane Laerryn Coramar-Seelie instructed Calum Staffwright to reject the request, even though she did not know why it was doing so. Shortly after, most of the Ring of Brass went to the Tree of Names to investigate it further. When they did so, they found that objects sent across the planes were caught in its branches. This included Evandrin, who was sent across the planes by Laerryn years ago as a test for the Astral Leywright. Patia Por'co attempted to learn more about it by casting Legend Lore and, in doing so, learned its true purpose through visions of the past. Loquatius Seelie tried to pull Patia away from the tree, which was growing around the hand she used to touch it, and saw into the Feywild. Lady Elmenore, noticing this, told him that he must prevent Patia from finding the names.

Although Nydas attempted to stop her, Laerryn cast Blight on the tree, breaking it in two, after it hurt Patia and Loquatius to defend itself from Patia trying to learn its secrets. Vespin then emerged from the destroyed tree, followed by Asmodeus. The subsequent explosion from the tree's destruction severed Patia's arm at the elbow, destroyed several of the party's magical items, killed Patia and Loquatius, and knocked Zerxus and Nydas unconscious. After its destruction, devils flooded Avalir. The tree's rapid destruction was confirmed to be possible due to it being weakened after Laerryn refused the energy requisition and shorted the tithe.

Evandrin later told Zerxus while the two were on the Astral Plane that he became anchored to that plane during Laerryn's experiments, and the tree seemed to see him as an intruder, pulling him back to that plane when he tried to return to Exandria.

Trivia

 * In Latin, "calix" means "chalice" and is derived from the Ancient Greek "κύλιξ" (kylix). So, together "Arboreal Calix" indicates a tree-like chalice, or a chalice for a tree. Additionally, in botany, the calyx is a cup-shaped group of leaf-like sepals at the base of a flower. This is derived from the Latin "calyx", which in turn is derived from the Ancient Greek "κάλυξ" (kálux); "calix" and "calyx" are distinct words in Latin, though they are used interchangeably in botanical Latin.
 * The Tree of Names is similar to a world tree, a recurring mythological motif of a tree that connects the heavens, the terrestrial world, and the underworld. The Norse Yggdrasil is one such world tree, and the Nine Worlds exist around Yggdrasil in a similar way that all planes are close to the Tree of Names.