The most powerful object in Mesopotamian myth is a piece of baked clay with a seal impression on it, worn on a cord against the chest. Akkadian calls it the ṭuppi šīmāti, the tablet of fates or destinies. Whoever wears it decides what happens.
That a civilisation which ran on sealed clay documents should imagine ultimate power as a sealed clay document is not a coincidence, and it is not a joke either. In Babylonian legal practice a transaction became binding when it was written and sealed; the seal is the authority, and the tablet is the act. Transposed to heaven, the same instrument does the same job at the scale of the cosmos.
What it actually does
A distinction that most retellings blur: this is not a record of the future. It does not contain a text that says what will happen, and nobody reads it to find out what is coming. It is an instrument of office — possession of it confers the power to decree, to fix what will be. It is closer to a seal of state than to a book of prophecy, which is why the plots involving it are about custody rather than about interpretation.
Its normal holder is Enlil, whose characteristic power is exactly the irrevocable decree. In first-millennium Babylonia it drifts towards Nabu, the god of the stylus, which makes obvious institutional sense: the document ends up with the scribe.
Stolen twice
The first theft is the plot of the Epic of Anzu. Enlil's own doorkeeper watches him set the tablet aside to bathe, takes it, and flies to the mountains — and the gods discover that they cannot simply send someone after him, because he can now issue commands that bind his attackers. The crisis is procedural rather than military, and the solution found by Ninurta is a technicality in the wording of a spell. That episode is followed in full on the Anzu page.
The second is in Enuma Elish, and it is a legitimation rather than a robbery. Tiamat fastens the tablet on the breast of her consort Kingu when she appoints him commander, saying that his command shall not be altered and the word of his mouth shall be fixed — which is the formula normally used of Enlil. When Marduk wins, he takes it from Kingu, seals it with his own seal, and fastens it on his own breast. The sealing is the crucial gesture and is easy to skim past: an instrument is transferred by re-sealing it in the new holder's name, exactly as a Babylonian legal document would be.
The dais of destinies
The object had a counterpart in practice. Babylon's Esagila contained a room called the chamber or dais of destinies, and at the New Year festival the gods were understood to assemble there and fix the fates for the coming year — twice, on separate days of the eleven-day cycle, with the king's own fate determined as part of it. The mythological tablet and the ritual chamber are two expressions of the same idea: that the future is set on a specific occasion, by an authorised body, in a designated room, and then holds.
Not the same thing as the me
Popular accounts frequently merge this object with the me — the divine offices and institutions that Inanna carries off from Eridu. They are different in kind. The me are the functions of civilisation themselves: kingship, the priesthoods, the crafts, the arts of the scribe, truth, descent to the underworld, and around a hundred others, listed in Sumerian. They are not written on anything and they are not an authority to decide. Losing the me means losing the institutions; losing the tablet means losing the power to decide anything at all. One is the contents of a civilisation, the other is the office that governs it.
What people ask about it
Is it the ancestor of the Book of Life, or of the Norns' carving?
The comparison is worth making carefully. Books in which names are recorded do appear in later Near Eastern religion — the biblical book of life, and the tablets on which deeds are written in later Jewish, Christian and Islamic material — and a real family resemblance runs through the region's habit of imagining divine judgement as record-keeping. But those are ledgers of persons and acts; the Mesopotamian tablet is a warrant conferring authority. Norse fate, meanwhile, is carved or spun by the Norns and is a very different image again, and Greek fate is textile from beginning to end. What is distinctive here is that the instrument is transferable — it can be stolen, and the power goes with it, which no thread or ledger can do.
Was there ever a physical one?
No object identified as such has been found, and none should be expected — it belongs to the mythological furniture rather than to temple inventories. What did exist, in quantity, is the ordinary technology it is modelled on: sealed tablets, some enclosed in clay envelopes with the terms repeated on the outside so that tampering could be detected, and cylinder seals rolled across wet clay as signatures. Anyone handling a Babylonian contract was holding a miniature of this idea.
Why do the myths keep giving it to the wrong person?
Because the stories are about legitimacy, and legitimacy is best dramatised by showing it in the wrong hands. In both cases the usurper's claim is formally valid — the bird can genuinely issue decrees, and Kingu genuinely holds the office his queen conferred — which is what makes the situations dangerous rather than merely criminal. A tradition that imagined power as personal charisma would resolve these plots by proving the hero stronger. This one resolves them by transferring an instrument and re-sealing it, and it takes care to show the paperwork being done. The same institutional cast of mind runs through the whole pantheon, as the tradition overview sets out.