Illus supported the revolt of Basiliscus against Zeno, then switched sides, supporting the return of Zeno (475-476). Illus served Zeno well, defeating the usurper Marcian, but had contrasts with the dowage Empress Verina, and supported the revolt of Leontius, but the rebellion failed and Illus was killed.
Illus was an Isaurian, but the time and place of his birth are unknown. He is said to have held various offices under the Emperor Leo I (457—474), and to have been an intimate friend of Zeno, apparently before his accession. But we first read of him in Zeno's reign and in hostility to that emperor.
Basiliscus, brother of the empress dowager Verina, the widow of Leo, had expelled Zeno from Constantinople (475) and sent an army in pursuit of him under Illus and his brother Flavius Appalius Illus Trocondus into Isauria, where Zeno had taken refuge. The brothers defeated the fugitive emperor (July 476) and blockaded him on a hill called by the people near it "Constantinople" (Suidas, s.v. Ζήνων). Illus also captured Zeno's brother, Longinus, whom he considered a tool to keep Zeno under control.
During the blockade, Illus and Trocondus were instigated by the senate of Constantinople to support Zeno against Basiliscus, with whom they had fallen into odium and contempt; Illus himself was discontented with the usurper, as he had allowed the killing of the Isaurians who remained in the capital after Zeno's flight. So Illus and Trocundus were prevailed on by the promises and gifts of Zeno to embrace his side, and to march with united forces towards the capital. At Nice in Bithynia they were met by the troops of Basiliscus under his nephew and general Armatus; but he, too, was overcome, and Basiliscus, forsaken by his supporters, was dethroned and put to death (477).
Illus was sole consul in 478, and in 479 he was instrumental in crushing the dangerous revolt of Marcian, grandson of the Byzantine emperor of that name, and son of Anthemius, emperor of the West. Marcian had married Leontia, daughter of the late Emperor Leo by Verina, and sister of Ariadne, Zeno's wife. His revolt took place at Constantinople, where he defeated the troops of Zeno with the support of the mob, and besieged him in the palace. For a moment Illus wavered, but his failing courage or fidelity was restored by the assurances of an Egyptian soothsayer whom he patronised. Marcian's forces were corrupted by Illus; and Marcian himself, with his brothers Procopius and Romulus, was taken. The brothers escaped, but Marcian was sent, either to Tarsus in Cilicia, and made a priest in the church there, or to the foot of Papurius, or Papyrius, a stronghold in Isauria, then used as a state prison.
Trocondus, the brother of Illus, was consul 482; and Illus himself enjoyed the dignities of patricius and magister officiorum. He is said to have employed his power and influence well, and to have rendered good service to the state in peace as well as in war. He assiduously cultivated science and literature.
It was perhaps his literary predilections that made him the friend and patron of Pamprepius, for whom he obtained a salary from the public revenue, and to whom also he made an allowance from his private resources. Pamprepius was a native of Thebes, or, according to others, of Panopolis in Egypt, an avowed heathen, and eminent as a poet, a grammarian, and especially for his skill in divining the future. Pamprepius was hated both by Zeno and by the dowager empress Verina, and during the absence of Illus, who had gone on some business into Isauria, they banished him on a charge of attempting to divine future events in favour of Illus and against the emperor. Illus, knowing that his intimacy with him had been the real cause of his banishment, received him into his household, and, on his return to the capital, took him with him. The date of these events is doubtful: it is possible that they occurred before Marcian's revolt, though a later date is on the whole more probable.
As the weakness of Zeno's character made him jealous of all persons of influence and talent, it is not wonderful that the commanding position and popular favour of Illus rendered him an object of suspicion, and that the emperor in various ways sought to rid himself of him. The ambitious Verina, the dowager empress, was also his enemy, and formed a plot against his life. The assassin, an Alan, employed by her, is said to have wounded Illus; but this is doubtful, as historians have confounded her plot with the later one of her daughter Ariadne. At any rate Verina's attempt was defeated, and Zeno, equally jealous of her and of Illus, banished her at the instance of the latter, and confined her in the fort of Papurius. There is some doubt as to the time of these events also. Candidus places the banishment of Verina before the revolt of Marcian, and Theodore Lector assigns as the cause of it her share in the revolt of Basiliscus. It is not unlikely, indeed, that this turbulent woman was twice banished, once before Marcian's revolt, for her connection with Basiliscus, and again after Marcian's revolt, for her plot against Illus. From her prison she managed to interest her daughter Ariadne, the wife of Zeno, in her favour, and Ariadne endeavoured to obtain her release, first from Zeno, and then from Illus, to whom the emperor referred her. Illus not only refused her request, but charged her with wishing to place another person on her husband's throne. This irritated her; and she, like her mother, attempted to assassinate Illus. Jordanes ascribes her hatred to another cause: he says that Illus had infused jealous suspicions into Zeno's mind which had led Zeno to attempt her life, and that her knowledge of these things stimulated her to revenge. The assassin whom she employed failed to kill Illus, but cut off his ear in the attempt. The assassin was taken, and Zeno, who appears to have been privy to the affair, was unable to prevent his execution.
Illus, with his friend Pamprepius, now retired from court, first to Nice, and then, on pretence of change of air and of procuring the cure of his wound, into the East, where he was made general of all the armies, with the power of appointing the provincial officers. Marsus, an Isaurian officer of reputation, who had first introduced Pamprepius to Illus, and the patrician Leontius, a Syrian, and an officer of reputation, either accompanied him or joined him in the East, and probably also his brother Trocondus. Having traversed Asia Minor they erected the standard of revolt (483 or 484). Illus declared Leontius emperor, defeated the army of Zeno near Antioch, and having drawn over the Isaurians to his party and obtained possession of Papurius, released Verina, and induced her to crown Leontius at Tarsus, and to send a circular letter to the imperial officers at Antioch, in Egypt, and the East by which they were prevailed on to join Illus. This important service did not, however, prevent Illus from sending Verina back to Papurius, where she soon after closed her restless life. Zeno (485) sent against the rebels a fresh army, said to consist of Ancient Macedonians and Scythians (Tillemont conjectures, not unreasonably, that these were Ostrogoths) under John "the Hunchback", or, more probably, John "the Scythian", and Theodoric the Ostrogoth, who was at this time consul. John defeated the rebels near Seleuceia (which town of that name is not clear, perhaps the Isaurian Seleuceia) and drove them into the fort of Papurius where he blockaded them. In this difficulty Trocondus attempted to escape and gather forces for their relief, but was taken by the besiegers and put to death. Illus and Leontius were ignorant of his fate, and, encouraged by Pamprepius, who gave them assurance of his return and of ultimate victory, held out with great pertinacity for above three years. In the fourth year the death of Trocondus was discovered, and Illus, enraged at the deceit practised on him by Pamprepius, put him to death. The fort was soon after taken by the treachery of Trocondus's brother-in-law, who had been sent for the purpose from Constantinople by Zeno, and Illus and Leontius were beheaded (488) and their heads sent to the emperor.
Tillemont and Le Beau regard the revolt of Illus as an attempt to re-establish heathenism; but for this view there seems no foundation. We do not know that Illus was a heathen, though Pamprepius was one: it is more likely that Illus was a man of no fixed religious principles, and that his revolt originated either in ambition, or in a conviction that his only prospect of safety from the intrigues of his enemies and the suspicions of Zeno was the dethronement of the emperor. It is remarkable that Edward Gibbon does not mention the name of Illus, and scarcely notices his revolt.