Q: Context: A will or testament is a legal document by which a person, the testator, expresses their wishes as to how their property is to be distributed at death, and names one or more persons, the executor, to manage the estate until its final distribution. For the devolution of property not disposed of by will, see inheritance and intestacy.

Question: who has the ability to ensure a last will and testament is followed as it is written?
Let's give stream of consciousness below
To answer this question, we should know that: A will or testament is a legal document by which a person, the testator, expresses their wishes as to how their property is to be distributed at death, and names one or more persons, the executor, to manage the estate until its final distribution. So the answer is the executor.