Wiktionary
mirror will
n. (context legal English) A pair of identical documents executed by two people in order to insure identical dispositions of their property upon the death of either or both.
Usage examples of "mirror will".
Since the mirror will show me any part of the Temple on request, I can use it to keep track of Mikayla when I'm here, instead of having to scry her personally or bespeak her lammergeier.
The mirror will remain silent We will not use it -- and neither of you can.