Unstoppable Expanding Sphere
No creature, object, or force can prevent the sphere from expanding. As the sphere expands, everything is Pushed outward and held at the edge of the sphere. Walls and buildings crumble as they are pushed outward (along with their debris). Even the ground beneath the caster is pushed downward, compressing, fusing, or crumbling under the fantastic force.
If the Sphere encounters an unmoving Force effect, such as Wall of Force, it makes an opposed potency check against the effect. If it succeeds, the force effect is destroyed and the sphere continues moving. If it fails the sphere's expansion is temporarily stopped by the force effect. Check again on the following segment. The sphere gets a cumulative +5 power bonus on its potency roll against the force effect for every additional check it makes.
Creatures pushed by the sphere suffer no other harm, unless they are pressed into a structure or similar blocking terrain. In that case the creature suffers Bludgeoning damage equal to the Durability of the structure, as it is crushed into the structure before it breaks apart. A creature pushed into multiple structures suffers this damage multiple times. The exact effects of the expansion are up to the GM, but a creature that cannot escape from being crushed into other objects (and their debris) will eventually be reduced to nothing more than a smear unless they are more resilient than what they are pushed into.
The area inside the sphere is devoid of matter aside from the caster and the small amount of air that initially surrounded him. After the first segment, treat the area as an empty Vacuum. This may have effects on the caster if he is not protected in some way. The only way past the barrier of the sphere is to teleport, open a portal, or travel via some other trans-dimensional means (an open portal may allow enough air or other substance to enter the sphere to negate the vacuum effect at the GM's discretion).