The sound mind requirement
In order to make a valid will, you must not only set out your wishes in a written and witnessed document, but you must also have, in the eyes of the law, the mental capacity to do so. This means you must make your will with "understanding and reason" and not be suffering from mental conditions such as delusion, insane suspicion or aversion.
It is your mental condition at the time you made your will is that legally relevant. If you suffer from any mental disorder, it is important that evidence is left with your will (for example, from a doctor) that proves you were mentally competent at the time you made the will. Otherwise, your will can be open to challenge.
Your will can also be challenged on the basis that you were acting under pressure or undue influence when you made it so it is important that you get independent legal advice and not use the services of a solicitor of any potential beneficiary of your will.