Русско-англо-русский юридический словарь

countersuit: an opposing lawsuit

especially : a claim asserted by an opposing party in a lawsuit (such as a defendant) against the party bringing the original lawsuit (such as a plaintiff)

//decided to file a countersuit against her accuser (Merriam-Webster Dictionary)

If a tenant sues their landlord for unfairly evicting them, the landlord might file a countersuit claiming months of unpaid rent. (Vocabulary.com)