Widespread interest in the story of Laocoön, a mythical priest of Troy, developed after an ancient, monumental sculpture representing him and his two sons was unearthed in 1506 in Rome. Suspecting trickery, Laocoön had warned his countrymen not to accept the wooden horse left outside Troy by the Greeks and had hurled his spear at it to prove that it was hollow. Thus the priest incurred the wrath of the gods, for desecrating an object dedicated to the goddess Athena. El Greco depicted serpents, sent by the angry gods, engaging Laocoön and one son in a mortal struggle, while a second son lies already dead at his father's side. The identity of the unfinished figures on the right continues to be debated; perhaps they represent the gods themselves supervising their vengeance.
Utilizing every available means — writhing line, lurid color, and illogically conceived space — the artist projected an unrelieved sense of doom. The figures seem incorporeal; sinuous outlines and anti–natural flesh tones contribute to their specterlike appearance. The striking setting carries this visionary late work of El Greco to an apocalyptic extreme.
Did El Greco intend to relate this mythical theme of conflict and divine retribution to the Inquisition then raging in Toledo? Whatever the case, the story of Laocoön is the only classical theme he is known to have painted.
More information on this painting can be found in the Gallery publication Spanish Paintings of the Fifteenth through Nineteenth Centuries, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/spanish-painting-15th-19th-centuries.pdf