In this paper, I define a religious-political phenomenon called cultist terror groups. A cultist terror (ct) group is a nationalist insurgent organization which employs extreme, ritualized violence, and establishes a cultist doctrine, commonly borrowing elements from established religions. IS, or the Islamic State, is the latest example of a cultist terror group. This paper examines and contextualizes IS by analyzing death cults throughout modern history, and contrasts these case studies with violent, religious, political groups that do not meet the heuristic characteristics of a cultist terror group. Cultist terror groups are so named to drive home the idea that these groups belong to a subset of the broader, religious terror (rt) group category. The paper aims to discern the key components of cultist terror groups that distinguish these groups from other violent, religious groups with political goals (a political theology). These include identity and strategy components. Specifically, it presents a concise set of hypotheses about how cultist terror groups create in-group solidarity (e.g., through synchronous activities), recruit members, prevent defection, select targets, and how these aspects differentiate cultist terror groups from other violent religious organizations with political theologies.