Demons were points, rivers were lines, and seas were planes.
Of the three defense lines, the Demon Sealing Defense Line within Donghua Mountain lay deep in the heartland of the Southern Kingdom, yet it was surrounded on all sides by the great clans and major sects, making it a solitary point of independence.
The East Sea Defense Line, meanwhile, had been squeezed apart into separate sectors by the territories of forces such as the Rakshasa Sect, Flying Soul City, Sword Discussion Pavilion, and the Sea Merchant Guild. When circumstances were favorable, it was like an enormous millstone, grinding all heavenly demons, Outer Dao cultivators, and foreign creatures into powder. But when circumstances turned bad, everyone fought their own battles, swaying in the wind and rain, enough to make one's heart tremble with fear.
As for the Cang River Defense Line, it began at Boundary Breaking Mountain and ran straight through to the East Sea, cutting across nearly the entire Eastern cultivation world. Its purpose was to defend against the increasingly chaotic Northern Demonic Tribulation. Almost the entire Southern Kingdom served as its rear, and only with an unceasing stream of powerful cultivators from among the Southern Kingdom's countless cultivators could they sustain that endless defensive front stretching hundreds of millions of miles.
The most threatening aspect of the void passage discovered by the patrol group was this: once foreign fiends found this place, or another similar location, the blockade the great clans and major sects had imposed around Donghua Mountain would collapse without a fight. An endless stream of demons and vicious men would charge out, leaving the Cang River Defense Line vulnerable to attack from both front and rear. The relatively stable rear of the Southern Kingdom might also suddenly collapse beneath such an assault.
Not one of the