It's just that the 'Fast' model picks up from a library of existing tracks using a small embedding model.

If you're really curious, you could try the 'Custom LLM' feature - specifically 'Gemini Flash'. Here's the track it created when I gave it the same input.

It titled it, "Throne of the Eternal Night" and here's the justification it came up with, "A glacial, minor-key procession of heavy brass hits and deep sub-pulses, evoking the suffocating grandeur of an ancient, blood-soaked dynasty ending in shadow."

Music link: https://vocaroo.com/14bW1RJyubcC