Thresher Maws are colossal, subterranean creatures that can be found on multiple planets in the Mass Effect universe. These gargantuan worms spend their entire lives either eating or seeking food. They can and will devour almost anything and can even gain sustenance from mineral ores and solar radiation. Thresher maws are solitary life-forms: they do not live or hunt in groups and there are rarely any more than three threshers to be found on a single planet. They reproduce via spores that can lie dormant for millennia and are robust enough to survive prolonged periods in deep space and atmospheric re-entry. As a result, thresher maw spores appear on many worlds, spread by previous generations of space travelers.


Thresher maws are primarily found on large asteroids or on planets that are unsuitable for supporting carbon-based life such as humans. It is speculated that the beasts originated from the Krogan homeworld of Tuchanka, though this is unconfirmed. They are highly territorial, however, and will relentlessly attack anything that moves through their expansive nest areas.


As previously stated, thresher maws are constantly seeking food and will attack anything that moves. It should be noted that the body of a thresher maw has never been fully witnessed, most of it remaining below ground. When threshers attack, they will swipe with their two clawed limbs or simply try to engulf prey in their tremendous mouthes. They can also expel a highly corrosive substance that eats through just about anything. They launch this acid as projectiles with substantial physical force, strong enough to penetrate kinetic shields altogether.


Kalros is the ancient "mother of all thresher maws" described in Krogan legends. During the Reaper war of Mass Effect 3, it was revealed that Kalros made its home in the deserts of Tuchanka where an ancient Krogan city once stood. It is the largest thresher maw ever witnessed and said to be at least as old as the city ruins it guards. During the Tuchanka campaign of the Reaper War, Commander Shepard was able to activate an ancient maw hammer device to summon Kalros, which attacked and buried a Reaper Destroyer.