Basilic

The Children of Life and Death believe Basilic is the language of the gods.
This language has cropped up periodically throughout history. The Den of Papyrus has found evidence of Basilic centuries before the Children of Life and Death formed.

Dreams & Knowledge

All known prophets of Basi received the gift of Basilic in a dream. Their waking mind unwilling or unable to arouse their body for a week, sometimes more.
Upon rising, the new prophet usually beckons for a writing tool to impart their encounter. There is evidence that at least one prophet wrote on their skin when no writing parchment could be found.
After these events, some prophets withdrawal and struggle with daily life. Some prophets chalk the visions and language up to a flight of fancy or an 'episode' and return to work. The rest are recruited by the Keepers of the Prophecy, a small organization within the Children of Life & Death.
Only a dozen or so prophets are ever found in a century. Without confirmation, the Keepers do not know how many prophets they've lost. Typically they'll only know of a missing prophet when the Den of Papyrus or Weaver's Guild reports an errant piece of Basilic writing.
Some Keepers of the Prophecy wonder if the Sect of the Spider Crushers ever recruit these missing prophets but cannot confirm or deny these concerns.

Famous Texts

The two most famous Basilic texts are the Tome of Life and Death and the Manifesto of the Lost Tome.
The Tome of Life and Death was written by one the Keepers' first prophets. The book spans Basi's values, life perspectives, and spell work. This grimoire and holy book were stolen from the Keepers of the Prophecy by the Sec of the Spider Crushers, cementing the tension between these two organizations.
The Manifesto of the Lost Tome was left behind in place of the holy grimoire. The script of the manuscript pulsed with the shimmering glow of a fresh enchantment. As it was read, the prophet who transcribed the text into common fell into a dreamless half-life for over a decade.

Still Weaving by Self

Edits

Replace Still Weaving by Self with an image of Basilic writing. Maybe a transcription of a page in the Tome or the manuscript?
Reread for edits
Emphasize that this language cannot be taught in the waking world.  It can only be bestowed upon you in a dream.  GMs, if your player tries to learn this language, it slips from their mind.  Think about a mind where you forget your last sentence.  Think about a mind where you forgot your last sentence.  Think about  a mind that forgot what it just thought.  What about when your mind forgets what you just said.  you get me