Ritual of twin burnings

The ritual of twin burnings was a complex and extremely evil arcane evocation spell or binding ritual that obliterated the body and spirit of a sacrificial victim. Its sole purpose was to create runes of chaos around Thakorsil's Seat, in order to permanently bind a creature to that artifact and enslave them to the caster's will.

Components
The key material component of the ritual of twin burnings was the sacrificial victim: a human mage of good nature, commanding magical power to a level in accordance with the number of runes of chaos inscribed.

In addition, it required some verbal and somatic components, among them the intonation of "words of dark terror". There were other material components costing 50,000 gp. These included a sacrificial knife with which to slay the victim.

Ritual
The ritual of twin burnings was not exactly a spell, but shared many similar features. It was quite a complicated ritual    that took a full night and day to perform in full. If the one performing it was disturbed at any point, then the ritual failed and had to be repeated in full. There were three main requirements.

Firstly, the ritual of twin burnings could only be performed during the full moon, and only once a month. If the ritual failed, then the practitioner had to wait a full month for the next full moon.

Secondly, it required the presence of Thakorsil's Seat itself, sitting no more than 20 feet (6.1 meters) away. It could only work with the Seat; if attempted with any other artifact, or the Seat was nowhere near, then the ritual had no effect.

Finally, it required the sacrifice of a good human mage of a certain level of power. The power of the mage required depended on the number of runes of chaos inscribed. Inscribing the ninth rune required a mage nine times more powerful than the beginning mage required to inscribe the first rune.

The ritual culminated in the caster intoning "words of dark terror", then slaying the victim with a sacrificial knife. At this time, runes already on the Seat played deep and eerie musical chords. At the very end, the victim was utterly destroyed in both body and soul—these were the twin burnings of the ritual.

Effects
If the ritual of twin burnings was completed successfully, then the very essence of the sacrificial victim was used to create a rune of chaos,  which was inscribed on a triangular crystal pane that appeared around Thakorsil's Seat. Each successive ritual created another rune of chaos and another crystal pane. A failed ritual did not destroy existing crystal panes, but they could be destroyed with only a little damage. Another completed ritual of twin burnings was required to replace broken panes.

With nine successful rituals, a nine-sided crystal pyramid was completed around the Seat, with a rune of chaos on each face. At this point, the occupant of Thakorsil's Seat was permanently and fully enslaved and bound to the will of the caster who had performed the rituals.

Involving the horrific sacrifice and eternal destruction of good beings, with the ultimate goal of permanently binding and enslaving a creature to one's will, the ritual of twin burnings was naturally a despicably evil act. Those who enacted it were either already utterly evil, or were inevitably turned evil in the process.

History
The ancient kingdom of Yhalvia (part of the empire of Narfell, −946 DR to −150 DR) was ruled by the devil lord Orlex, but a band of rebellious wizards arose to depose him. They created the throne known as Thakorsil's Seat to imprison and permanently bind and enslave the evil fiend. Unfortunately, although they had the best of intentions, the device demanded deeds of despicable evil to function. Thus, they devised the ritual of twin burnings, which involved the sacrifice of beings of good heart, believing this was for the greater good, that the ends justified the means.

They went ahead with the horrific rites, and successfully bound and enslaved Orlex in the Seat, forming a council of wizards that governed Yhalvia. However, corrupted by the sacrifices and with Orlex as their slave, they soon became as cruel, depraved, and wicked as the devil they had replaced.

At some point, Thakorsil's Seat—and presumably the details of the ritual of twin burnings too—came into the possession of Larloch, the Shadow King of Warlock's Crypt, who never actually made use of it.

Eventually, Szass Tam, the Zulkir of Necromancy of Thay, visited Warlock's Crypt around 1366 DR. The two came to some deal or alliance, and Larloch gave Szass Thakorsil's Seat, the Death Moon Orb, several other powerful magic items and artifacts, a number of hooded companions, and presumably the details of the ritual of twin burnings, to aid him in his plots to control Thay and the demon Eltab.

Szass then embarked on his scheme to bind the demon lord Eltab. Centuries before, Eltab had aided the Thayans in shaking off Mulhorandi rule, but was subsequently imprisoned beneath the capital, Eltabbar. Szass Tam had realized that the demon lord would soon break free, and decided to instead hasten his escape and then fully enslave the fiend. In the Year of the Shield, 1367 DR, Szass Tam used the Orb's power to first free Eltab then compel him to sit in Thakorsil's Seat, which immediately imprisoned him again. He then began the terrible task of performing the ritual of twin burnings and creating the runes of chaos to make Eltab his eternal slave. During this time, the Death Moon Orb hovered in place over Eltab in Thakorsil's Seat in the Chambers of Twin Burnings.

Nathor, a Thayan civil servant, bore witness to much of Szass Tam's wicked deeds, sitting at his left hand as he watched a ritual of twin burnings. Nathor later fled to Aglarond in horror.

However, before Szass Tam could inscribe the ninth and last rune of chaos, a band of mighty adventurers got into the Chambers and foiled his scheme. All the crystal panes and the runes of chaos were destroyed and Eltab escaped.

Lore
By the 1360s DR, only Larloch the Shadow King of Warlock's Crypt and Szass Tam, Zulkir of Necromancy of Thay, knew the details of the ritual of twin burnings. It was not recorded in any written form.