The Sacrifice Box by Martin Stewart

Sep, Arkle, Mack, Lamb and Hadley: five friends thrown together one hot, sultry summer. When they discover an ancient stone box hidden in the forest, they decide to each make a sacrifice: something special to them, committed to the box for ever. And they make a pact: they will never return to the box at night; they'll never visit it alone; and they'll never take back their offerings.

Four years later, the gang have drifted apart. Then a series of strange and terrifying events take place, and Sep and his friends understand that one of them has broken the pact.

As their sacrifices haunt them with increased violence and hunger, they realise that they are not the first children to have found the box in their town's history. And ultimately, the box may want the greatest sacrifice of all: one of them.