Cyaxares king (r. 625-585 BC) chose as his capital the city of Ecbatana (present-day Hamadan, Iran). In 625 he drove the Scythians out of Media and imposed his rule over the Persians. He attacked the Assyrians next and captured (614 BC) the city of Ashur. In 612, in alliance with the newly independent kingdom of Babylonia, he captured the city of Nineveh and overthrew the Assyrian Empire. Thereafter Cyaxares extended the territory of his kingdom to include all of eastern Anatolia. Cyaxares was succeeded by his son Astyages (r. about 584-c. 550 BC). The Persians, under Cyrus the Great, revolted against him about 550 BC. Joined by a portion of the Median army under a chief named Harpagus, they took Ecbatana and deposed the Median king. From that time Media was politically subservient to Persia; the Persians, however, regarded the Medes as equals, and thenceforth the two peoples were considered as one.
Sorry for lack of material, but there is no written documents in Median preserved. From Old Persian and Assyrian sources we can find some not numerous words, mostly personal names, of Median. These sources gave linguistics a chance to distinguish some characteristic features of phonetics and morphology of the language.
Now we can state that Indo-Iranian *s' / c' > s (while in Old Persian it was u), Indo-Iranian *s'u > Median sp, and *su > f, *tr > vr. But this is the only thing by which we can judge Median was not a dialect, but a single Iranian language.