This backpack has a secret interior space only accessible to the creature attuned to it that can hold up to 125 pounds, not exceeding a volume of 16 cubic feet. The backpack weighs 5 pounds, regardless of its secret contents. Retrieving an item from the secret part of the backpack requires an action.
Every time an item is put inside, the attuned creature can mentally choose if it will be secret, making it inaccessible to everyone else.
If the backpack is turned inside out, only its public contents spill forth, leaving the secret content unharmed, but the backpack must be put right before it can be used again. Breathing creatures inside the backpack can survive up to a number of minutes equal to 3 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.
Placing a backpack of secrets inside an extradimensional space created by a Heward's handy haversack, portable hole, a bag of holding or similar item or vice versa if using the secret compartment instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it to a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.