Your Patron has granted unto you a final resting place, your own private sanctum, and a remembrance of your own mortality. Select a location where your character could have feasibly visited in the past. That becomes the entrance to your tomb. You determine the verbal and somatic prerequisites for entrance, if any, such as a secret passphrase that turns a mundane doorway into a portal, or the correct path through a clutter of trees to slip into the extra dimensional space. Alternatively, the entrance may simply be mundane.
Regardless, the Tomb is 15 cubic feet, designed in a manner to your liking. While inside, time passes normally, but you do not age and do not require food, drink, or sleep. Creatures other than you cannot benefit from Short or Long Rests while within your tomb. All items left in your Tomb remain in the Tomb, in the exact state you left them, until you retrieve them. This Tomb may be considered your home plane for the purposes of spells like Banishment (your choice).
You may change the location of the entrance to a place within 30 feet of you and the prerequisites for entrance to your Tomb via a 1 hour ritual.