Hysiae was

Hysiae (Argolis) .

Pausanias 9.1.6, 9.2.1:

But Neocles, who was at the time Boeotarch at Thebes, not being unaware of the Plataean trick, proclaimed that every Theban should attend the assembly armed, and at once proceeded to lead them, not by the direct way from Thebes across the plain, but along the road to Hysiae in the direction of Eleutherae and Attica, where not even a scout had been placed by the Plataeans, being due to reach the walls about noon.

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On Mount Cithaeron, within the territory of Plataea, if you turn off to the right for a little way from the straight road, you reach the ruins of Hysiae and Erythrae. Once they were cities of Boeotia, and even at the present day among the ruins of Hysiae are a half-finished temple of Apollo and a sacred well.

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Hysiae (Boeotia)

Pausanias 2.24.7, 8.54.7

....On coming down to a lower level you reach the ruins of Hysiae, which once was a city in Argolis, and here it is that they say the Lacedaemonians suffered their reverse

...

Mount Parthenius rears also tortoises most suitable for the making of harps; but the men on the mountain are always afraid to capture them, and will not allow strangers to do so either, thinking them to be sacred to Pan. Crossing the peak of the mountain you are within the cultivated area, and reach the boundary between Tegea and Argos; it is near Hysiae in Argolis.

An Argive border citadel S of the modern village of Achladokampos on the road between Lerna and Tripolis. The town was destroyed by the Lakedaimonians in 417 B.C.; following the defeat, the Argive dead were buried at Kenchreai. The ruins of Hysiai were seen by Pausanias and the walls were described by Curtius as polygonal on ashlar foundations, and flanked by round towers.

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A town in Boeotia, east of Plataeae, called by Herodotus (v. 74) a demus of Attica, but probably belonging to Plataeae.

Herodotus (Histories Book 9)

His army at this time lay on the Asopus, and stretched from Erythrae, along by Hysiae, to the territory of the Plataeans. The wall, however, was not made to extend so far, but formed a square of about ten furlongs each way.

Accordingly, with a large army, he invaded the district of Eleusis, while the Boeotians, who had concerted measures with him, took Oenoe and Hysiae, two country towns upon the frontier;

Historical events

Battle of Hysiae (or Hysiai), c. 669 BC Sparta defeated by Argos

In the winter 417-416 the Spartan King Agis II captured Hysiae.

Battle of Plataeae