Gherridarigaaz was a powerful Shivan dragon who allied with Urza on Dominaria.
Gherridarigaaz died sacrificing herself to protect Urza from a quick and powerful blast of Radiant's spell during the battle in Serra's Realm.
Earlier, her son Rhammidarigaaz had chosen to be the viashino's champion and she had been angered at that decision. The viashinos had an agreement with Urza to stop her from attacking them so that Urza could use the rig. Urza then threatened her.
Rhammidarigaaz would later become leader of the Dragon Nations and turned out to be reincarnation of one of the godlike Primevals.