From their announcement:
After the success of our 10mm Dungeon Packs, we’ve put together a handy Booster Pack, with extra rooms for your Heroes to explore, and even more Monsters for them to watch out for.
The Booster Pack contains 10 resin pieces creating 2 extended rooms, 6 pieces of furniture, and 9 extra monsters. All for the bargain price of £15.99.
Keep an eye out in the future for all kinds of Dungeon-y bits and bobs, including themed Monster packs.