The Tribe of Levi weren't counted among the twelve tribes. The tribe of Levi was named for Jacob's son Levi. Interestingly, the Tribe of Levi did not have a designated portion of the Canaan, as it as given the special purpose by God as the priestly tribe. It is distinct from the Levitical priesthood or the Office of Levi in the tribe is the family/inheritance entity itself, rather than priestly functions it members had to conduct. Many notable people arose from the tribe of Levi, such as Moses and Aaron.
Revenge against Shechem
The Tribe of Levi all descended from the patriarch Levi, the third son of Jacob and Leah. During Levi's adult life, his sister Dinah was raped by Prince Shechem of the Hivites. After this Shechem asked Jacob to have her hand in marriage. Levi and all of his brothers were furious, so they tricked the entire civilization to circumcise themselves. Levi and his brother Simeon were particularly angry. In vengeance, Levi and Simeon lead an armed force into the city and killed every male; also looting the entire city of its treasures. When Levi and Simeon were confronted by their father, they responded that Dinah should not have been treated like a prostitute.
Levi had three sons Gershon, Kohath and Merari. During his lifespan he and all of the Family of Israel took refuge in Egypt. At this time Levi only had his three sons with no other grandchildren. At the end of Jacob's lifespan in Goshen, Egypt he gave final words to all of his sons. While most of the sons received good blessings, Levi's family was given a curse: that they would be scattered and dispersed throughout Israel.
While in Egypt the Tribe of Levi began to grow. Each of Levi's three sons had their children. Each of these children became the progenitors of their respective clans, totaling to eleven clans. Amram, the head of his clan married Jochebed another Levite, and they bore Moses, Aaron and Miriam.