My wizards has both flaming sphere and bigby's icy grasp, which are both conjurations. According to the PHB "a conjuration can't be attacked or physically affected" (moved). Can I use a conjuration to block a passageway?
My DM was saying that they've changed the rules for conjurations in PHB3? Can a conjuration be attacked or moved out of the way?
The answer to this is:
Conjurations do not by default occupy squares and therefore cannot block passageways.
There are exceptions to this rule. If the power says it can, it can.
Conjurations do not by default take actions, and cannot permit opportunity attacks for passing them because they are not creatures.
There are exceptions to this rule. If the power says it can, it can.
Conjurations cannot be attacked except by abilities that explicitly target conjurations.
There are exceptions to this rule. If the power says it can, it can.
Here's the problem with the general question you've provided; if we give you the RAW answer, you'll go and look at the conjuration you're thinking of, and the answer could be wrong because -that- conjuration uses an exception to the rules.
Conjurations are notorious that way.
Fortunately, you've provided to examples that we can actually utilize to answer -specificly- to your question.
FLAMING SPHERE:
Effect: You conjure a Medium flaming sphere that occupies a square within range, and the sphere attacks.
According to the latest rules update for Flaming Sphere, it occupies the space, so yes, it DOES block the space and prevent enemies from travelling through it.
As an added bonus, it cannot be attacked by most attacks, so you've got a barrier of fire that cannot be easily destroyed.
BIGBY'S ICY GRASP:
This power does not occupy a space. Therefore, enemies can walk right through it. As well, the enemy you grasp with it will not block movement for its allies.
It, like the Flaming Sphere, cannot be attacked by normal attacks however.
