"the coming of the fiery Trigon"
The four elements - water, fire, air and earth.
Creative Commons AttributionThe four elements - water, fire, air and earth. - Credit: Gunjan Karun on Flickr

In astrology, the signs of the zodiac are separated into four groups, corresponding to the four classical elements from which it was believed everything in the world was made: fire, earth, air and water. Each group is called a Trigon, and contains three signs. Aries, Leo and Sagittarius belong to the fiery Trigon. Taurus, Virgo and Capricorn make up the earth Trigon. Gemini, Libra and Aquarius form the air Trigon, and Cancer, Scorpio and Pisces belong to the water Trigon.