About Deotarius, Bishop of Arisitum
There is almost no information on Deotarius, but there does exist some information on the Bishopric of Arisitum (Ales).
From the English Wikipedia page on the Ancient Diocese of Alais:
About 570, Sigebert, King of Austrasia, created a see at Arisitum (Alais) taking 15 parishes from the Diocese of Nîmes. In the 8th century, when Septimania was annexed to the Frankish Empire, the Diocese of Alais was suppressed and its territory returned to the Diocese of Nîmes.
Ben M. Angel notes: Prior to 719, the Diocese was probably functioning, but likely went dormant in that year, when the Moors swept into Septimania (the southern coast of France). According to the Wikipedia page on Septimania:
The Moors, under Al-Samh ibn Malik the governor-general of al-Andalus, sweeping up the Iberian peninsula, by 719 overran Septimania; al-Samh set up his capital from 720 at Narbonne, which the Moors called Arbuna, offering the still largely Arian inhabitants generous terms and quickly pacifying the other cities.
Following the conquest, al-Andalus was divided into five administrative areas roughly corresponding to Andalusia, Galicia and Lusitania, Castile and Léon, Aragon and Catalonia, and Septimania.
With Narbonne secure, and equally important, its port, for the Arab mariners were masters now of the Western Mediterranean, he swiftly subdued the largely unresisting cities, still controlled by their Visigoth counts: taking Alet and Béziers, Agde, Lodève, Maguelonne and Nîmes.
By 721 he was reinforced and ready to lay siege to Toulouse, a possession that would open up Aquitaine to him on the same terms as Septimania. But his plans were overthrown in the disastrous Battle of Toulouse (721), with immense losses, in which al-Samh was so seriously wounded that he soon died at Narbonne.
Arab forces soundly based in Narbonne and easily resupplied by sea, struck eastwards in the 720s, penetrating as far as Autun (725). But in 731, the Berber wali of Narbonne and the region of Cerdagne, Uthman ibn Naissa, called "Munuza" by the Franks, who was recently linked by marriage to duke Eudes (also called Odo) of Aquitaine, revolted against Córdoba, and was defeated and killed.
In October of 732, an Arab force under Abdul Rahman Al Ghafiqi encountered Charles Martel between Tours and Poitiers, and was defeated. This "Battle of Tours" (also called the Battle of Poitiers) is celebrated in popular history and traditionally credited with stopping the Moorish advance in Europe.
After the territory round Toulouse was taken by the Franks in 732, Pippin III directed his attention to Narbonne, but the city held firm in 737, defended by its Goths and Jews under the command of its governor Yusuf, 'Abd er-Rahman's heir. (Note: Palladius was known to be Bishop of Nimes, the eventual successor diocese to Arisitum, in 737, while Gregorius succeeded as Bishop sometime before 745.)
Around 747 the government of the Septimania region (and the Upper Mark, from Pyrénées to Ebro River) was given to Aumar ben Aumar. In 752 the Gothic counts of Nîmes, Melguelh, Agde and Béziers refused allegiance to the emir at Córdoba and declared their loyalty to the Frankish king—the count of Nîmes, Ansemund, having some authority over the remaining counts. The Gothic counts and the Franks then began to besiege Narbonne, where Miló was probably the count (as successor of the count Gilbert) But Narbonne resisted.
In 754 an anti-Frank reaction, led by Ermeniard, killed Ansemund, but the uprising was without success and Radulf was designated new count by the Frankish court.
About 755 Abd al-Rahman ben Uqba replaced Aumar ben Aumar. Narbonne capitulated in 759 and the county was granted to Miló, the Gothic count in Muslim times. The region of Roussillon was taken by the Franks in 760. In 767, after the fight against Waifred of Aquitaine, Albi, Rouergue, Gévaudan, and the city of Toulouse were conquered.
In 777 the wali of Barcelona, Sulayman al-Arabi, and the wali of Huesca Abu Taur, offered their submission to Charlemagne and also the submission of Husayn, wali of Zaragoza. When Charlemagne invaded the Upper Mark in 778, Husayn refused allegiance and he had to retire. In the Pyrenees, the Basques defeated his forces in Roncesvalles (August 15, 778).
The Frankish king found Septimania and the borderlands so devastated and depopulated by warfare, with the inhabitants hiding among the mountains, that he made grants of land that were some of the earliest identifiable fiefs to Visigothic and other refugees. Charlemagne also founded several monasteries in Septimania, around which the people gathered for protection. Beyond Septimania to the south Charlemagne established the Spanish Marches in the borderlands of his empire.
(Note: The Diocese of Nimes had probably absorbed Alais by this time, as Sesnandus became Bishop of Nimes, and he was followed by a regular succession of bishops thereafter.)