Aulis is a small Greek port city in Boeotia. It is important because of its role in the Trojan War. The Oxford Classical Dictionary says Aulis, the best harbor in northern Boeotia, was near Tanagra and located on a rocky peninsula between two bays.
The Greek forces led by Agamemnon, set out for Troy but mistakenly landed in Teuthrania, land of King Telephis, son of Hercules. Telephus set them in the right direction. For their second voyage, they set out from the port of Aulis where Agamemnon made his fateful sacrifice to Artemis of his daughter Iphigenia.
The seer Calchas had told Agamemnon that Artemis was angry and that the only way he would be able to appease her was to sacrifice his daughter Iphigenia. With the conniving of Odysseus, and perhaps Diomedes, Agamemnon arranged to have Clytemnestra bring Iphigenia to Aulis, ostensibly to marry Achilles, but in reality to be sacrificed for the war effort.
The Achaeans led by Agamemnon weren't the only Greeks to use Aulis as a launching point. Military leader Agesilaus stopped in Aulis before heading to Asia and the poet Hesiod set sail to Euboea from Aulis. Aulis features in the diadoch Antigonus' conlict with Cassander.
John Buckler "Aulis" The Oxford Classical Dictionary. Simon Hornblower and Anthony Spawforth. © Oxford University Press 1949, 1970, 1996, 2005.
Trojan War People• Helen of Troy
• Dioscuri - Castor and Pollux