The ancestral tunic was left to the chosen one by his ancestors. It is one of the four magical symbols that the heir must gather. It allows Twinsen
to store magical points.
In LBA, it is found in a closet in Twinsen's house. In LBA2, the ancestral tunic and Sendell's medallion are combined in one item, and they are found in Miss Bloop's Private Museum.
LBA: "With this magical symbol, you have just earned the first level of power."