IRS Form 56
IRS Form 56 is the Notice Concerning Fiduciary Relationship.
This officially designates the person who will be responsible for filing tax returns and paying taxes on behalf of the deceased taxpayer's Estate.
This form is also used again, after the estate is finally settled and closed, to notify the IRS that the fiduciary relationship has ended.
The process of managing a deceased person's taxes can be complicated, so it's best to speak with Certified Public Accountant to help navigate the process.