Short answers, pulled from the story.
The Heruli were one of the smaller Germanic peoples of Late Antiquity, appearing in records from the third to sixth centuries AD. Their ultimate origins are disputed; many scholars believe they originated in Scandinavia and migrated south to the Black Sea before the third century, while the linguist Alvar Ellegård argued they were a warrior brotherhood that formed north of the Danube in the late third century.
In 267, a Heruli fleet departed from the Sea of Azov, passed through the Bosphorus, seized Byzantion and Chrysopolis, and raided islands in the Aegean before landing in Greece. They plundered Sparta, Corinth, Argos, and the sanctuary of Zeus at Olympia, then sacked Athens despite a wall built only a generation earlier during Valerian's reign. Local militias defended the city and the writer Dexippus became famous for his role in that defense.
The Herulian kingdom north of the Middle Danube was destroyed by the Lombards in the period 493-511 AD. King Rodulf was killed in battle against the Lombards around 494 according to Procopius, but the kingdom may have continued somewhat later given Theoderic of Italy's correspondence with Herulian royalty preserved in the Variae of Cassiodorus.
The Heruli are believed to have spoken a Germanic language, though the evidence is contested. Given their association with the Goths, they may have spoken an East Germanic language related to Gothic; given proposed connections to Scandinavia, a North Germanic language has also been proposed. Walter Goffart argued that most personal names of Herulian leaders baffle German philologists, while Prostko-Prostyński argued that a large number of Herulian names are certainly Germanic.
Pharas was a notable Herulian commander who led a cavalry unit under Belisarius in the Iberian War against the Persians in 530 and then in the campaign against the Vandals in Carthage in 533-534, where he played a very prominent role in the Roman victory. Several thousand Heruli served in the personal guard of Belisarius throughout his campaigns, and in 552 a commander named Philemuth led 3,000 mounted Heruli in the conquest of Rome.
The Heruli disappear from the historical record around the late sixth century. Sinduald, executed by Narses in 567, is the last historically attested Herule. The Herulian federate polity near Belgrade vanished from surviving records shortly after, apparently absorbed by the incoming Pannonian Avars, while Slavic languages were becoming common in the Middle Danubian region.