The city of Acragas was founded in 580 BCE by people from Gela, a Greek town on Sicily that had been founded more than a century before by Rhodians and Cretans. The new city rose to great power when Phalaris, the son of Leodamas of Rhodes, became its tyrant ("sole ruler").
According to the philosopher Aristotle of Stagira, writing two centuries later, Phalaris first occupied an important office, which was probably the construction of the temple of Zeus Polieus in the citadel of the city. According to a later story, told by Polyaenus, Phalaris armed his laborers and occupied the citadel. It may be true, but an attack with an army of foreigners on the citadel is more or less what every tyrant did, and the story may have been modeled on the histories of other tyrants.
Phalaris is credited with other construction projects too: he strengthened the walls of Acragas, built an aqueduct, and added several other beautiful buildings. He also improved trade with Carthage and organized impressive athletic contests. All this is plausible. In a new city like Acragas, we can expect construction works, economic measures, and the establishment of festivals.
The tyrant is also credited with war against native cities - one of them was Vessa - and capturing them. Their territories were added to that of Acragas. In other words, Phalaris embarked upon an expansionist foreign policy. This may have culminated in an appointment as military commander of Himera on the northern coast of Sicily, something mentioned by Aristotle. But the great philosopher may simply have confused the names of Acragas and Himera.
After sixteen years, Phalaris was overthrown in a general rising headed by a man named Telemachus.
Phalaris became the prototype of an evil dictator. One of the most famous stories about him is that of the iron (or brazen) bull made by Perilaus of Athens, a well-known bronze-worker. The victims of the tyrant were shut up inside this artifice and then a fire was kindled underneath the bull. The poor prisoner was now roasted alive, and by some sort of acoustic mechanism, the shrieks of his agony resembled the bull's bellowing. The first victim is said to have been Perilaus himself; the last one Phalaris.
The story was clearly invented to suggest what tyranny was all about, but there may be an element of historical truth. Writing less than a century after Phalaris, the poet Pindar assumes that everybody knows the story of the bull. Human sacrifice was not unknown in Carthage and western Sicily, and it may be noted that in this age, Sicilian rivers were often represented as bulls with human heads. We can no longer reconstruct what really happened in Acragas, but we cannot exclude the possibility that two stories were conflated - one about human sacrifice in a Carthaginian village near Acragas, and one about a monumental statue dedicated by Phalaris to a river.
Two accounts of the story of the bull of Phalaris can be found here.