Looking at the grammar of the phrase in the question, it's somewhat ambiguous if it's a non-restrictive appositive or a dependent clause. Which it is depends on how you interpret it—and it's the use of the comma that makes it ambiguous.
As a non-restrictive appositive
Apposition is a grammatical construction in which two elements, normally noun phrases, are placed side by side, with one element serving to identify the other in a different way; the two elements are said to be in apposition. One of the elements is called the appositive, although its identification requires consideration of how the elements are used in a sentence …
Here and elsewhere in this section, the relevant phrases are marked as the appositive phraseA or the phrase in appositionP …
A non-restrictive appositive provides information not critical to identifying the phrase in apposition. It provides non-essential information, and the essential meaning of the sentence would not change if the appositive were removed. In English, non-restrictive appositives are typically set off by commas. The sentences below use non-restrictive appositives.
- Alice SmithP, my friendA, likes jelly beans. – The fact that Alice is my friend is not necessary to identify her.
- I visited CanadaP, a beautiful countryA. – The appositive (that it is beautiful) is not needed to identify Canada.
- The first to arrive at the houseA, sheP unlocked the front door.
If I analyze the sentence in the question as a non-restrictive appositive, and use the same notation as the Wikipedia article, it would look something like this:
The length of these reigns may suggest that both of these kings are actually demigodsP, drawn from mythology rather than historyA.
The article that I quoted says that the two elements in apposition are normally noun phrases, and that wouldn't necessarily preclude a verb phrase being an appositive. But I've been unable to find an example of an appositive that's a verb phrase rather than a noun or noun phrase, so I can't say with certainty that it can actually be looked at in this way.
However, with the specific phrase in this question, it could still be interpreted as an elided noun phrase:
The length of these reigns may suggest that both of these kings are actually demigodsP, [figures] drawn from mythology rather than historyA.
If the missing noun figures is assumed, then it's a noun phrase, and the appositive analysis is correct.
As a dependent clause
If it's not interpreted that way, but as a verb phrase, then what follows the comma becomes a dependent clause.
You can reverse the relevant portions of the sentence to see this:
Drawn from mythology rather than history, both of these kings are actually demigods.
What makes this interpretation interesting, is that if the phrase really is a dependent clause, common style guidance would have there be no comma in the original version.
In other words, the complete sentence would look like this:
The length of these reigns may suggest that both of these kings are actually demigods drawn from mythology rather than history.
The comma makes it ambiguous
In short, based on common style guidance, the existence of the comma suggests an elided noun phrase acting as a non-restrictive appositive. But if it's interpreted as a dependent clause instead, then there would normally not be a comma.