Definition & Meaning of "instrumental-comitative case"

What is the "instrumental-comitative case"?

The instrumental-comitative case is a grammatical case used to indicate both the means by which an action is performed and the accompaniment of someone or something in that action. It combines the functions of the instrumental case, which expresses "by means of," and the comitative case, which expresses "together with." This case is found in languages such as Uralic languages like Mari. For instance, in Mari, a noun would change form to reflect both the tool used and the companionship during an action. It helps clarify both how and with whom an action is carried out.