Suspiria: Everything You Need To Know About The Three Mothers
2. How They've Been Defeated Before
When the Three Mothers commisioned Varelli to construct the three homes for them, he was unaware of their evil powers. And when he did find out, it was too little too late. But, he did write the Three Mothers book to chronicle his encounters with them and to hopefully help someone defeat them one day.
In each of the three films, the Mother's power was linked directly to their respective houses. Thus, if the house was destroyed, so too was the mother, and vice versa. In Suspiria, Suzy descends into the catacombs beneath the Tanz and defeats Mater Suspiriorum by stabbing her in the throat. As a result, her entire coven dies and the Tanz implodes around them.
In Inferno, Mark Elliot is only able to defeat Mater Tenebrarum by destroying her entire building and allowing it to collapse around her. And finally, in Mother of Tears, Sarah Mandy destroys Mater Lachrymarum's most powerful relic, a magical cloak, and her entire building collapses on top of her and her army of followers.
In other words, Varelli's construction of the buildings for them was unwittingly each Mothers' own downfall.