Sendell's medallion is one of the four magical symbols the heir must gather. In LBA, it is found in the secret room in Twinsen's house, along Gawley's horn, where his ancestors left it for him. In LBA2 Sendell's medallion and the ancestral tunic are combined in one item, and they are found in Miss Bloop's Private Museum.
When Twinsen finds Sendell's medallion in LBA, he gains the third level of power: the red magic level.
Inventory Description[]
"Sendell's Medallion. With this magical symbol, you have just earned the third level of power"