Apuleius initiation

Yes. Apuleius gives the famous description in The Golden Ass, Book XI, especially chapters 21–24—but there is an important wrinkle: he also describes two subsequent initiations into the mysteries of Osiris and Serapis later in Book XI.

1. The Isis initiation —

Metamorphoses

XI.21–24

This is the passage you are probably thinking of.

The sequence is:

• XI.21 — The priest Mithras explains that initiation occurs only when Isis herself indicates the appropriate time. Apuleius calls initiation “a kind of voluntary death and salvation obtained through prayer.”

• XI.22 — Lucius undergoes purification, receives instructions from the priest, and prepares himself.

• XI.23 — Ten days of abstinence from meat and wine; then, at night, he is taken into the innermost sanctuary. This is the extraordinary passage in which Apuleius deliberately refuses to tell the reader exactly what happened. He says he approached the boundaries of death, passed through the elements, saw the sun shining at midnight, and came into the presence of the gods.

• XI.24 — At dawn he emerges dressed as an initiate, carrying a torch and wearing a solar/palm crown, and is displayed before the congregation.

This is therefore not really a description of the secret ceremony itself. It is a description of the experience and external framework surrounding it. Scholars are quite cautious about trying to reconstruct the actual rite from Apuleius.

2. There is actually more in Apuleius: the Osiris/Serapis initiations

This is particularly interesting.

At the end of Book XI, after Lucius has gone to Rome, he undergoes another initiation, this time associated with Osiris.

Then he undergoes yet another initiation. Traditional editions consequently divide the final section roughly as:

XI.28–30 — initiation into the mysteries of Osiris and Serapis

The old table of contents of the English edition explicitly describes these as the “Second and third ceremonies of Initiation to the Mysteries of the Gods Osiris and Serapis.”

So Apuleius actually gives us something like:

Isis → Osiris → another Osirian/Serapian initiation

This is one reason Book XI is much more complicated than simply “Lucius gets initiated into Isis.”

3. Is there another ancient author who describes the Isis initiation?

Not in anything approaching Apuleius’s narrative form.

The other major ancient source is Plutarch’s De Iside et Osiride (On Isis and Osiris). It is probably the most important text to read alongside Apuleius. The Metropolitan Museum identifies these two works—Plutarch and Apuleius—as the two most informative literary sources for the Isis cult, although both provide rather general descriptions of initiation.

But there is a fundamental difference:

Plutarch explains the mythology and theology of Isis and Osiris; Apuleius gives us the experience of the initiate.

And this makes Apuleius extraordinarily valuable for the question you’ve been pursuing with Eliade: what does a mystery initiation mean phenomenologically?

Apuleius gives us several striking motifs:

purification → fasting → sacred instruction → entering darkness → symbolic death → passage through the elements → seeing the sun in darkness → encountering the gods → emergence/rebirth → new identity.

That is remarkably close to the sort of death-and-rebirth / sacred-time / passage between cosmic levels structure that Eliade later analyzes comparatively.

There is also a fascinating caution here: Apuleius explicitly withholds the central secret. He tells us what the initiate experienced, but not what the priests actually did. Modern scholars therefore distinguish between what Apuleius actually says and reconstructions based on Eleusinian or Egyptian parallels.

If you want to pursue this, I think the really fruitful next step would be to put Apuleius XI.21–24 beside Plutarch, Isis and Osiris §§25–27 and then compare both with Eliade’s discussion of initiation in Rites and Symbols of Initiation. That produces a surprisingly clear three-way comparison: ancient ritual → ancient interpretation → modern phenomenology.