Rending asunder is a phrase that indicates a violent or forceful separation or splitting of something. Some synonyms that can be used in place of rending asunder include tearing apart, splitting, breaking up, shattering, fracturing, and disintegrating. These words are often used in the context of physical objects, but they may also refer to the separation of relationships or the breaking of agreements. It's worth noting that each of these synonyms implies a different degree of force or violence, so the choice of word will depend on the specific context of the situation being described.