Sebastian is a shadowy figure in Fifth Century AD history, who operated in the chaotic 430's and 440's. F. M. Clover attempted a chronological outline of Sebastian's career in the paper, 'Count Gainas and Count Sebastian' which appeared in The American Journal of Ancient History, Issue 4, 1979, pp.65-76. I think he more or less got it right, though one might quibble over some of the dates, and I will use his outline to discuss Sebastian's activities.
Sebastian first emerges in the historical record after his father-in-law, Count Boniface, defeated Aetius at the battle of Ravenna (Rimini) in 432 AD. Boniface died soon after from wounds sustained in the battle, leaving Sebastian to succeed him as Comes et Magister Utriusque Militiae (essentially, 'supreme commander') of the Western Empire. Sebastian did not hold this position for long, as very soon after, Aetius returned and, with support from the Huns, seized control of the Western Empire, by 433 AD at the latest.
Fleeing East, Sebastian arrived in Constantinople. What happens next is unclear, but it seems Sebastian wore out his welcome at Court. It has been suggested that Sebastian and his supporters were plotting to overthrow the current regime. Sebastian was forced to flee Constantinople and he and his men began making piratical attacks in the Hellespont and Propontis.
Around this time the seaward defences of Constantinople were improved. Some believe this to be a response to Vandal raids, or at least the threat of raids. However it is wrong to assume that all Mediterranean pirate activity in this period was conducted by the Vandals. We know of at least one other pirate, named Contradis, who was captured and executed in about 438 AD. It seems it was in response to both his threat and that of Sebastian that Constantinople sought to improve its defences. The Vandals did not raid in the East until later.
In Clover's chronology by either 438 or 439, Sebastian and his men had fled West again to the court of Theoderic I, King of the Goths. It is worth noting that Clover's dating here differs from others. The chronicler Hydatius dates Sebastian's arrival in the Gothic court as being 444 AD. However, Clover, to my mind, argues convincingly for the earlier date citing the chronicler Prosper Tiro and linking Sebastian's movements persuasively to contemporary events.
The arrival of Sebastian in Toulouse, along with the fall of Carthage to the Vandals (439) and the subsequent Gothic-Vandal alliance presented Aetius and the Western government with a formidable problem. Aetius marched into Gaul and defeated the Goths in battle, forcing Theoderic to accept a renewal of the treat of 418, which had established the Goths as foederati.
Sebastian and his men were forced to move again, this time to Barcelona which they captured. From here they sailed to Africa, pitching up at the court of Geiseric. This was their last stop as Geiseric eventually had Sebastian executed (the date is uncertain, following Hydatius it was about 450 AD).
The Christian writer Victor of Vita described Sebastian's fall from grace and execution. What is perhaps most remarkable about his account is that he devotes so much time to a man who clearly was not very religious. Every other 'martyr' against the Arian Vandal regime mentioned by Victor has their virtue and piety expressed in detail. Sebastian is referred to as a valiant fighting man, but little is said of his religious belief. It seems Sebastian's death, to Victor at least, was an important event and one worth discussing, even if Sebastian did not conform to the standards of the usual victims of the Vandals.
Victor suggests that Sebastian was extremely popular at court and that Geiseric felt threatened. Whether Sebastian ever stood a realistic chance of seizing control of Carthage and the Vandal state is uncertain. One would suspect not, but if prominent Vandals were beginning to see him as a potential replacement for Geiseric, it would possibly say something about the multi-ethnic nature of the Vandal force. It should be remembered that the Vandal army also contained Alans, Goths and Romans. Even if the Hasdingi and Siling Vandal elite were loyal to Geiseric, there may have been a significant number of commanders from the other minority ethnic-groups who viewed Sebastian as a valid candidate.
Another reason for Geiseric ordering Sebastian's death may be that Aetius demanded it as part of the alliance agreed between the Romans and Vandals. Plausible, but if so why was Sebastian only executed in 450? It is possible that Hydatius' dating is erroneous here and the event took place earlier, or that there may have been another reason altogether for Sebastian's death.
A picture of Sebastian emerges from all of this. He seems to have been a charismatic, daring and dangerous individual who was seen wherever he went as being a potential threat to established regimes. This tells us that even after Aetius deposed him that he still wielded considerable authority and power. Sebastian had a number of troops (bucellarii) with him. The actual numbers may have fluctuated over time but their presence suggests Sebastian had significant resources with which to maintain them.
Once Sebastian was in Constantinople he would have soon found himself in a quandary. To keep his men loyal and attached to him he would require funds, but the Imperial government, one easily imagines, would be reluctant to fund a large, armed fighting force belonging to one so ambitious. Driven out of Constantinople, Sebastian's piracy was probably undertaken not so much as an attack on Constantinople, but as a means to reward his followers and to maintain his household.
The resources of the Eastern Empire were such that Sebastian would soon have realized that continued piracy in the East was very dangerous. Therefore he returned to the West, perhaps with the intention of gaining Gothic support to defeat Aetius and win back his former position. The Gothic defeat put paid to this and so Sebastian moved to Barcelona. This attack was primarily to pillage and reward his soldiers. Sebastian must have realised, however, that his options were running out.
He needed an ally who would support and shelter him or turn a blind eye to any piratical attacks he launched to support himself. Preferably an ally who was opposed to Aetius. There was only one option, Geiseric.
Dr. Clover argues that Sebastian arrived in Carthage around 440 while Geiseric was in Sicily. The Vandals had captured Marsala and were besieging Panormus. No mention is made in the sources of whether Panormus was captured, but their silence hints, to me at least, that the Vandal siege failed. It could be speculated that Geiseric was forced to abandon the siege, or at least curtail it, upon receiving the news that Sebastian and his faction had arrived in Carthage. This forced Geiseric to return to Africa to confront Sebastian.
Clearly an understanding was reached between Geiseric and Sebastian, one which held until Geiseric either decided to remove the threat to his position or was persuaded by Aetius to do him a favour.
Sebastian achieved little during his career and could be described as a mere pirate or brigand. But he was symptomatic of the times. A man such as himself could only operate and, for a time, be successful in an environment where imperial power structures were collapsing. In this power vacuum men like him could wreak havoc, playing competing factions against one-another and preying on those who were left defenceless by Rome's weakness. It is worth remembering that until the defeat of the Goths, Sebastian probably had a real chance of regaining his position and ousting Aetius. Afterwards, however, he was always going to become marginalised and this in turn led to his eventual death.
Reading: Clover, F. M., 'Count Gainas and Count Sebastian', in The American Journal of Ancient History, Issue 4, 1979, pp.65-76. See also Heather, P., The Fall of the Roman Empire, for a good overview of the period. Primary sources are hard to obtain, look for Hydatius, Victor of Vita, Count Marcellinus and Prosper Tiro. All are mentioned in Clover's work.