Having lost the war to the Spartans, the citizens of Athens elected thirty men as oligarchs.
The Thirty introduced oppressive and highly exclusionary laws and began a political purge against Athenians who had been Spartan informers and collaborators during the long war.
Eventually open hostilities between the Thirty and disenfranchised or disaffected Athenian citizens led to a coup d'etat in 403 BC that deposed them.
However, the age of the Tyrant does not fit with the context of the latter speech: he must have been aged at least 18 in 411 BC to have served as a trierarch, and at least 30 by the time of the Thirty Tyrants period in 403/2 BC.
But "On the Murder of Eratosthenes" suggests that the Eratosthenes referred to in that speech was younger than 30 at the time of his death – he is described as neaniskos, a word which Lysias elsewhere uses to refer to younger men.