According to the Magic Circle spell description, you can reverse the spell while casting it so that a creature of a chosen type is trapped in the circle (without a saving throw) and creatures outside of it are given some pretty good buffs.
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the Cylinder and protecting Targets outside it.
- The creature has disadvantage on Attack rolls against Targets within the Cylinder.
- Targets within the Cylinder can't be Charmed, Frightened, or possessed by the creature.
Although the circle itself has a range of 10-feet, the buffs do not have a range in the case of an inverted circle. Is it essentially just infinity then? This seems... exploitable.