Mithridates I Ktistes: first king of Pontus (r.281-266 BCE).
The beginning of Mithridates' career is mentioned by Diodorus of Sicily, although it is a bit complex.
At about this time Mithridates, who was subject to Antigonus but appeared to be shifting his allegiance to Cassander, was slain at Cius in Mysia after having ruled that city and Myrlea for thirty-five years. Mithridates, inheriting the kingdom, added many new subjects and was king of Cappadocia and Paphlagonia for thirty-six years.note[Diodorus, World History 20.111.4; tr. Russel M. Geer.]
Cassander and Antigonus had been colonels of Alexander the Great and had created kingdoms of their own: Cassander in Macedonia, Antigonus in Asia. The first-mentioned Mithridates, the man that was slain, had been ruler in Mysia from 337 to 302. The second Mithridates, who inherited the kingdom, was his nephew, the son of a man named Ariobarzanes. The family descended from an earlier ruler, also called Ariobarzanes, who had governed Mysia from 363 to 337.note[Diodorus, World History 15.90.3.] The family claimed to belong to the highest Persian nobility.note[Polybius, World History 5.43.2; Diodorus, World History 19.40.2.]
Several other sources confirm that Antigonus suspected the Mithridatic Dynasty. For example, Plutarch tells us that the younger Mithridates was a courtier of king Antigonus, but that the king came to suspect him after a dream. The king's son, Demetrius, helped Mithridates escape. The story may be topical (it closely resembles the Biblical story of Jonathan and David), but confirms that Antigonus suspected the Mithridatics.note[Plutarch, Life of Demetrius 4; cf. Appian, Mithridatic Wars 9.]
The younger Mithridates fled to the mountains of Paphlagonia, which "he used as a base of operations when he established himself as lord of Pontus", as Strabo tells us.note[Strabo, Geography 12.3.41.] Again, Plutarch confirms this.note[Plutarch, Life of Demetrius 4.] In the next decades, he could consolidate his position, because the successors of Alexander needed local supporters to build their kingdoms. He appears to have collaborated with the Greek city of Heraclea.note[Memnon, Fragment 7.2.]
After the death of two of these successors, Lysimachus and Seleucus, in the year 281, Mithridates accepted the royal title, which he left to his son Ariobarzanes. Later generations called him Ktistes, "the founder", of Pontus, and remembered him as "a man remarkable for courage and trained from childhood as a soldier".note[Diodorus, World History 19.40.2.]