The Nameless Quest in The Gate of the Sanctuary from The Temple of the Holy Ghost (Collected Works, Vol I) by Aleister Crowley.

“Yes, yes! let fancy sup
That grislier banquet than old Atreus9 planned!
Mind cannot fathom, nor the brain conceive,
Nor soul assimilate, nor heart believe
The horror of that Thing without a Name.
Full on me, boasting, like Death’s hand it came,
And struck me headlong.” [via]

Aleister Crowley notes on this text: 9. Atreus, King of Mycenae, gave a banquet of pretended reconciliation to his half-brother Thyestes, at which the two sons of Thyestes were served up.