Kalros

Kalros, a legendary thresher maw known as "the mother from which all other thresher maws spawn" and the largest thresher maw yet seen, plays a major part in the ongoing Reaper war of 2186. She roams Tuchanka's wastelands near the salarians' Shroud facility, acting as a natural deterrent to the vital installation. A mural found within the ancient krogan city nearby that Shepard explores prominently depicts Kalros, indicating that she is at least as old as the city itself.