How does Magic Circle against Evil work?

Question

Magic Circle against Evil^d20SRD has two primary effects:

1. Affecting all creatures within 10-ft. of its target with a Protection from Evil^d20SRD effect.
2. Drawing a 10-ft.-radius circle centered on its target that prevents trespassing of non-good creatures (either inward or outward).

My questions are:

• Once the spell is cast, should affected creatures remain within 10 ft. from the target in order to gain the effects?
• Does the 10-ft.-radius emanation follow the target as it moves?
• Does the drawn circle follow the target as it moves?

For now I'm not going to expose my opinion in order to avoid biasing the answer.

Answer 1

For the normal casting of Magic Circle, it follows the target and people only gain the benefits of the spell if they are within 10 feet of the caster. The protection is an "emanation" which means a continuously radiating effect. Other emanations are detect magic, zone of truth, silence, etc. - none of these affect things that have been affected and then left the area.

Yes, the emanation follows the target as it moves. Again, that's the way all these spells work (silence targeted on a creature being the canonical example). Nothing about the spell (or other spells with the same stats) says it doesn't move with the targeted creature.

For the "binding" version, no, the drawn circle does not follow the target - it's a physical circle drawn on the floor, not part of the spell effect. The whole point of the second version is to keep something from moving.

Really the second use of Magic Circle is an example of poor spell design; it's actually a completely separate spell that overrides a lot of the normal spell stat block (like you don't have to touch the summoned creature, the spell text makes it clear that it gets applied 'for free' with the summon/bind)...

Answer 2

Once the spell is cast, should affected creatures remain within 10 ft. from the target in order to gain the effects?

Yes. The spell is casted on one touched creaature (or self) and stays centered on this target. People in the area gain normal bonuses, and evil exteriors and elementals won't be able to enter the circle.

Does the 10-ft.-radius emanation follow the target as it moves?

Yes. As in protection from evil, if the character moves, he keeps the bonuses, and I guess the barrier moves with him. If there were to be an evil exterior in the way, I would make an epic "I must push you with my strength by the will of my spell" happen, with opposed strength rolls.

Does the drawn circle follow the target as it moves?

no. it's drawn. imagine the evil mage summoning an glabrezu in a candlelit room. he doesn't want the glabrezu to get out. and if he's good he probably permanenced the circle beforehand.