Amethyst pendant
A hollow amethyst on a chain that makes a container look empty
A proper necklace with a pale purple stone hung on it. The stone is hollow, missing half of what should be inside it, no matter how thick the walls look.
It came to Mikhail in a hidden compartment of the lute case Danil brought him, together with a scroll. The scroll holds no power of its own; it is an explanation, setting out what vacuous vessel is and the theory of how it works. The magic is in the pendant.
The spell is a small one. Touch a bag, a chest, a sack, anything no bigger than about a cubic foot for each level the caster has, and it looks and feels empty. What is inside is still inside; a hand reaching in will simply not find it, unless the person it belongs to is stubborn enough to know better. Cast it on something else, or put one more thing into the container, and it lapses.
The pendant will grant it at the cost of a spell the wearer had ready. That is the ordinary bargain, and for Mikhail it is not a bargain at all. Since the soul-goo he has cast the smallest spells without limit, and vacuous vessel is as small as they come.