The answer to the puzzle surrounding the true meaning of the Via Combusta was revealed by Al Biruni in his work, The Book of Instruction in the Elements of the Art of Astrology, written in 1029AD in Ghaznah. On page 317, number 514, he wrote:

"The combust way is the last part of Libra and the first of Scorpio. These two signs are not congenial to the Sun and the Moon on account of the obscurity and ill-luck connected with them and because each of them is the fall of one of the luminaries.

 They also contain the two malefics, the one by exaltation (Libra, Saturn) the other by house (Scorpio, Mars).

 The peculiarity however which has given the name muhtariq is that the exaltation of Saturn is near the fall of the Sun being on the one hand and that of the Moon on the other, while the adjacent parts of both signs are occupied by terms of Mars."

This area of the zodiac, which is now believed to be from 15 degrees Libra to 15 degrees Scorpio, (note that Al Biruni does not specify the degrees) is deemed to be malefic for a number of reasons: