Mana Vault

A cube of intricate metal, with a hinged door on one side. Even coming close to this object is enough to realize that the cube hums with immense power. A small glass orb on the door of the cube allows one to look inside, where the bright-blue glow of energy can be seen. Once per long rest, the door to the cube may be opened as a bonus action. When the door is opened, energy rushes outwards, filling every inch of your body. After this turn ends, you take an additional three turns in a row. After those turns have been taken, the door to the cube remains open, and the power flowing into you becomes dangerous and painful. At the end of each of your turns, you suffer one level of exhaustion, and take 2d8 radiant damage. This damage is not subject to damage resistance or immunity. You must spend four consecutive rounds taking no actions, bonus actions, or reactions to end the effect on yourself, though you may still move up to your speed. During these rounds, you do not suffer a level of exhaustion at the end of your turn.