She is only referred to by Tacitus in the `` Annals , '' where he tells of a grove in Frisia that was dedicated to her , near where 900 Roman prisoners were killed in 28 CE . Baduhenna is solely attested by Tacitus ' Annals where Tacitus records that a grove in Frisia was dedicated to her , and that near this grove 900 Roman prisoners were killed in 28 CE .