It was a small, waxy but smooth, pale yellow stone found only on Lorien 's first moon. When charged by a member of the Garde it turns a darker shade of yellow and when passed to another Garde, Legacies can be transferred for a period of around an hour.
Four takes the Xitharis with him when he, Six and Sarah travel to the Arkansas to find Number Five. Four gives it to Six so she can transfer her Invisibility to Four or Five. Later when the Mogadorians appear she tosses it to Five, who just stares at it and doesn't pick it up. Later, Five admits that he lost the stone and though the group try to find it, they had to give up the search.
Five later reveals to Ella that he stole the Xitharis. He charges it with his flight legacy and gives it to Ella so that she could fly away from the Anubis. She doesn't get a chance to use it but is presumed to still be in her possession.