"They looked like giant, speftral figures." is that a simile?

No, that is a description.

A figure of speech is a departure from the normal patterns of language for the purpose of emphasizing something. The simplest figure of speech is the SIMILE. A simile emphasizes a similarity of two things by merely saying it: "You are like a dog", or "You are as a dog". The figure rests entirely on one word.

A METAPHOR emphasizes a similarity of two things by saying they are the same; "You are a dog".

Next comes a big word: HYPOCATASTASIS. This is a Greek word for name-calling. Hypocatastasis just calls the fellow "Dog!" See Luke 13:32 "that fox", and Genesis 3:1 "the serpent".

A PARABLE is an extended figure of speech; a story based on a simile, metaphor, or hypocatastasis. If the story is possible, it is a MYTH. If the story is impossible, it is a FABLE. If a fable includes an explanation of the meaning, it is an ALLEGORY. Don't confuse any of these with LEGEND, which is a supposedly true but unverified historical account (Adam and Eve, for example).

These terms are not used with any precise meaning in modern discourse. For instance, most people think 'allegory' means "a story full of religious symbolism beyond human comprehension". But when discussing figures of speech they are very precisely defined. Here is a book that lists about 900 figures found in the bible. It is almost the only work in the subject for the last two thousand years: openlibrary.org/search?q=e.+w.+bullinger+figures+of+speech

More figures: mentalfloss.com/article/60234/21-rhetorical-devices-explained