Shaikh Zainuddin is better known as Shaheed-e-Thani (the second Shaheed), is among the greatest Shia Fuqaha. He was born in 911 AH. in Jabal Amel, but he must have lived in Tus for a considerable time, as he occasionally signed his name as "al-Tusi, al-Shami ".
He was a widely traveled man, having visited Egypt, Syria, Hijaz, Baitul Muqaddas, Iraq and Constantinople (Istanbul). Always in pursuit of knowledge, he studied from nearly twelve Sunni Ulama of Fiqh. Apart from the proficiency in Fiqh, he was well versed in Usul, Philosophy, Irfan, Medicine and Astronomy.
He was a man of piety, known for his austere way of life. His students have recorded in his biography that Shaheed maintained his family by selling the woods cut by himself during the nights, and then sat to teach during the day. While in Ba'lbak, he conducted classes in Fiqh according to five schools, i.e. Ja'fari, Hanafi, Shafei, Maliki and Hambali. His famous work is the commentary on al-Lum'ah which had been authored by Shaheed-e-Awwal. His commentary, Sharh-e-Lum'ah is a part of curriculum in almost every Hawza even today. He studied from Muhaqqiq Karaki before the later migrated to Iran.
Shaheed-e-Thani's son wrote the famous book on Usool, called Ma'alimul-Usool. Shaheed-e-Thani was martyred in 966 AH.