Mhach had some mentions in 2.x, though those were limited to the BLM questline (esp. Relic Weapon - a replica of Shatotto's old staff). Not much mention was made of Mhach except that it was part of the War of the Magi and developed Black Magic as well as used Voidsent. It got a little more presence in 3.0, with the SCH quest explicitly dealing with the Voidsent that infected Nym with the Tonberry plague.

Their amazing technological prowess (even if it's powered by Voidsent) and the full extent of their capabilities, however, was not made apparent until we set foot on the Void Ark.