(Canto 15, lines 109-114)
Priscian (full name Priscianus Caesariensis) was a Latin grammarian during the 5th-6th century active in Constantinople. His "Institutiones Grammaticae" in eighteen books was a standard Latin grammar for much of the Middle Ages. There is no historical evidence for his sodomy, only a notation in the works of 13th century poet Ugoccione da Lodi calling him a "fallen monk". This has led some to believe that Dante actually mixed him up with Priscillian, a 4th century heretic executed for sodomy among other "sins". Others have pointed out that the link between Priscian and sodomy is strenghtened by the common contemporary association of this sin with men of learning and schools.
Francis of Accorso or Franciscus Accursius (1225-1293) was a famous lawyer, like his father Accursius. He was raised in Bologna, where he became an instructor of civilian law at the university until 1273. He was then invited to England by king Edward I, where he came to teach at Oxford. Returning to Italy in 1281, his property, confiscated from him as a Ghibelline in 1274, was returned to him. He died in Bologna in 1293.
"That one" is Andrea dei Mozzi, born into a noble family and a chaplain to Popes Alexander IV and Gregory IX. Later he was a follower of Cardinal Latino and was sent as a representative of Pope Nicholas III to mediate the Guelf-Ghibelline conflict. He was a priest in Florence from 1272 and was promoted to bishop in 1287. It was during his time as bishop in Florence that work began on the famous Basilica of Santa Croce - construction of the subsidiary hospital is said to have been supervised by Folco Portinari, the father of Dante's love Beatrice. In 1295, Andrea was transferred by Pope Boniface VIII to Vicenza, where he died the same year (or in the beginning of the next). His transfer occasioned quite an uproar at the time, and since he was widely condemned for his scandalous life, it was even speculated that his transfer was caused by his brother Tommaso dei Mozzi, in order to rid Florence of his bad influence.
The "Servant of the Servants" is an appellation sometimes used for the Pope, here Boniface VIII. Arno is the river flowing through Florence, while Bacchiglione flows past Vicenza.