Departure from Egypt
Ptolemy was the eldest son of Ptolemy I Soter, Pharaoh of Egypt, and his first wife Eurydice, daughter of the regent Antipater. After Keraunos' younger half-brother, also called Ptolemy, was named heir apparent and, in 282 BC, ascended to the throne as Ptolemy II, he had to leave Egypt, being a potential rival for the throne. He arrived at the court of Lysimachus, the king of Thrace and Macedon, and part of Asia Minor, where his half-sister Arsinoe was queen.
While Ptolemy was staying in Lysimachus's court, Arsinoe's intrigues led to Lysimachus' first son, Agathocles, being accused of treason and then being executed. Keraunos sided with his other sister (from the same mother) Lysandra, who was Agathocles' wife, and accompanied her to the court of Seleucus in the East to solicit his aid. Seeing an opportunity to intervene for his own gain in the politics of both Lysimachan Thrace and Ptolemaic Egypt, Seleucus prepared an expedition against Lysimachus shortly afterwards.
King of Macedon
After Lysimachus' defeat and death in the Battle of Corupedium in 281 BC against Seleucus, Ptolemy Keraunos murdered Seleucus in order to gain control over the territories of his former protector. He then rushed to Lysimachia where he had himself acclaimed king by the Macedonian army. At this time he also formally relinquished his claim to the Egyptian throne.
To secure his position on the throne, Ptolemy asked his half-sister Arsinoe, the widow of Lysimachus, to marry him. Later in the year, he made an alliance with Pyrrhus of Epirus. His only rival, Antigonos Gonatas, son of the ex-king of Macedon, Demetrius I Poliorcetes, was confined in the city of Demetrias, Thessaly, and so Keraunos' power extended to southern Greece as well.
Arsinoe was not happy with the situation. While he was away on a campaign, she conspired against him from the capital, Cassandreia, with the aid of her sons. Keraunos quickly captured Cassandreia, and killed Arsinoe's two younger sons, while the eldest fled north to the kingdom of the Dardanians. Arsinoe herself fled to Egypt, where she married her own brother, Ptolemy II, and became Arsinoe II.
Although Ptolemy Keraunos was at the zenith of his power, he did not live long afterwards. In 279 BC, he was captured and killed during the wars against the Gauls led by Bolgius who conducted a series of mass raids against Macedon and the rest of Greece. Ptolemy Keraunos' death brought anarchy to the Greek states, since none of his successors were able to bring stability. This situation lasted about two years, until Antigonos Gonatas defeated the Gauls in a battle near Lysimachia, Thrace, in 277 BC. After this victory he was recognised as king of Macedon and his power extended eventually to southern Greece.
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