Ergative?

I have a case in my lang, Eviendadhail, which I used to think was
ergative, but, after reading more about ergativity, I'm now unsure.
This case indicates the thing used to perform the action.  For example,
I could say, without using this case:

        Éyaverog silen negeth.

which is:

        The boy killed the jabberwock.

Or, I could say:

        Ilelés éyaverog silen negeth.

which is:

        The boy killed the jabberwock with a sword.

This case indicates the sword, that is, the tool used to perform the
verb.  Does anyone know what this is called, if anything?

-M

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