Who comes down in one continuous line with Wudang in the north and south?

It's Zhang Sanfeng.

Zhang Songxi, the founder of Nanpai Taiji Biography, was a disciple of Zhang Sanfeng in Ming Dynasty and got the true biography of Zhang Sanfeng. He founded the Songxi School in Ningbo, Zhejiang, and later spread to Shanghai, Sichuan and other places. Tai Ji Chuan was founded by Wang Zongyue, a martial artist in the late Ming and early Qing Dynasties and a descendant of Zhang Sanfeng. He practiced martial arts as a family boxing in Shanxi, combined with the essence of Shaolin family boxing and other schools, and founded the biography of Wang Taiji.

The northern and southern Wudang come down in one continuous line, both of which originated from Wudang Neijia Boxing in Zhang Sanfeng, but each has its own characteristics and development. Zhang Sanfeng was a Taoist priest in the late Southern Song Dynasty and the early Ming Dynasty, and also the founder of Wu Tangmen Wushu. He became a monk in Wudang Mountain and later came back from the dead in Baoji. He also met with King Shu Xian of the Ming Dynasty. He was awarded the title of "Tongwei Jieshi" by the Ming Emperor.