This article aims to examine the development of ijmāʿ as a legal-theoretical source in in the third century after hijra, which has been labeled as the dark era in the history of uṣūl al-fiqh, by tracing back the ideas of different people and groups on ijmāʿ. It has tried to uncover these debates that were housed in independent treatises, in introductory chapters of certain works, and in the works of various disciplines written during this period, by referring both to the extant works falling into these three sets of sources and to the references made in the later literature of uṣūl al-fiqh to the scholars of the period who participated in these discussions. As a result of these discussions that can be classified under two rubrics as the validity and the nature of the ijmāʿ, it has been identified that the people and groups who had been cautious towards ijmāʿ before recognized ijmāʿ by assigning a great authority to it either by redefining the concept or abandoning their previous positions as of the end of the third century. It has been observed that scholars of ahl al-raʾy who can be referred to as the founding theorists of ijmāʿ, have also revised their original theories in the light of the criticisms directed to them. Finally, it has been discovered that the contributions made through reinterpretation and correction in this century have transformed into the types and ranks of ijmāʿ, and it was radically influenced the topic of ijmāʿ by the ways in which it was handled within the subsequent works in the literature of uṣūl al-fiqh.