“A scene of almost unspeakable beauty presented itself, lights of every imaginable colour were issuing from the southern heavens,” he would later recall.
I see here three clauses are added in a one single sentence without even adding no linkers. How is it possible?
Several remarks can be made here:
The man is talking (it is dialogue) and when you are talking you take all kinds of liberties. So, he omitted an and conjunction between the two sentences.
OR
You can view this as an apposition, there are two clauses that are in fact in apposition. The second one is like a restatement of the first.
“//A scene of almost unspeakable beauty presented itself//, lights of every imaginable colour were issuing from the southern heavens,” he would later recall.
An apposition in simplest terms is a restatement using a phrase or a clause and is set off by commas.
Most languages allow one to speak without always following some predetermined pattern.
I suggest you translate the sentences into your own language as dialog and you will see that.
["A scene of almost unspeakable beauty presented itself], [lights of every imaginable colour were issuing from the southern heavens],” he would later recall.
The two bracketed clauses are independent (main) ones, so strictly speaking the writer should have transcribed the original utterance using heavier punctuation than a comma to link them. A colon, semicolon or dash would do the trick.
Not a serious error, but easily avoided by a careful writer.
