She is in charge of religion. She holds a connection to the spirit realm and the pantheon of Goddesses. Every full moon, she holds a ceremony with the tribe to give thanks and ask for blessings from the Goddesses. She helps to maintain the public's love for the tribe by offering spiritual guidance for those that want it. She is required to be a witch. Black represents her role, so she always wears that color.
Araceli was banished by Queen Vivica because of her emotional manipulation, by the use of magic, of several ladies within the dynasty, which included the Queen. It is unknown if her intentions were to turn the Queen's Court against the Queen and take the throne. Luckily the Queen caught on early and took appropriate action.
Araceli's daughter, Crystal, will take over as Priestess when she comes of age.