I would suggest a binding. I use poppets for this myself - this one is to stop someone working against you, but you can modify the wording to bind them into firm adherence to your Will. Vovin’s Freeze spell is also a variation of a binding.
I believe he means a legal Will and testament: the executor is the person tasked with making sure it happens as written.
A will and testament is a legal document that expresses a person’s wishes as to how their property is to be distributed after their death and as to which person is to manage the property until its final distribution. - Wikipedia