As Grimm touches the bag, it immediately attunes to him.
As he looks inside the bag, an arm slips into the darkness.
The bag man is a creature that lives inside of your bag of holding that can potentially cause "VERY BAAD THIIINGS"
If he shakes the bag, Grimm knows that it will attack the closest thing.
Every time he finishes a long rest, Grimm must roll a 1d6.