Bag of the Swamp

The swamp gives, but the swamp also takes.

The bag of the swamp appears to the holder as a small hessian bag filled with small stones of various shapes and sizes, all with a faint glow to them. Picking one out will immediately trigger its effect. 

IRL, fill a bag with two sets of dice. One heals the user for the amount it rolls, the other damages them for that amount. The weilder can choose what dice it is based on feeling, but cannot see the colour of the dice before choosing.

Once a die is used, it does not return to the bag.

The healing dice cannot heal over your maximum HP, and damage dice work similarly to the Harm spell, in that it cannot reduce the target's hitpoints below 1.