In ergative-absolutive languages, the subject or agent of a transitive verb is marked with the ergative case.
This alignment groups grammatical roles differently from English. In a nominative-accusative system, the subjects of transitive and intransitive verbs pattern together, while the object of a transitive verb is treated differently. In an ergative-absolutive system, the subject of an intransitive verb and the object of a transitive verb share the absolutive pattern, while the transitive agent receives ergative marking.
For example, a language may mark “the man” differently in “the man saw the dog” and “the man slept.” In the first sentence, the man is a transitive agent and takes ergative marking; in the second, the subject is intransitive and follows the absolutive pattern. Basque and many languages of the Caucasus, Australia, and the Americas show ergative-absolutive features.
A common mix-up is choosing absolutive case because it also marks a subject—but only the subject of an intransitive verb. The precise role described in the question is the transitive subject, making ergative case the canonical answer.