Arcane Prison

You whisper of few quiet words, and a magical barrier molds itself into existence.  This barrier is in the shape of a sphere, which can be partially submerged under earth. The radius of the sphere starts anywhere from 5 feet to 15 feet. It is slightly visible as a ripple in the air, or the mirage that heatwaves create. 

This sphere can be entered from the outside, but creatures totally in it cannot move out of it. Also, the sphere acts similarly to an antimagic field. Magical items taking into the arcane prison disappear 5 seconds later, unless they were touching the skin of the person who brought them in. If that is the case, the effects work, but are restricted to the sphere. An item that teleports a character, for example, cannot teleport the character outside of the sphere. Spells are also restricted. They can be cast inside the sphere, but spells such as misty step and fireball  are stopped from leaving the sphere. 

As an action, the caster can change the size of the sphere. It can only extend or shrink to a size that has a radius of between 5 to 15 feet. If a Huge or Gargantuan creature is trapped inside a shrinking sphere, they take 8d8 bludgeoning damage and are moved outside of the sphere. If a group of creatures is too large for a shrinking sphere, each creature takes 1d10 bludgeoning damage and the outer ones are bumped out. Additionally, for every creature that has resistance or immunity to bludgeoning, another 1 bludgeoning damage is dealt to the rest of then creatures.

The spell can last for 24 hours unless the caster decides to end it. If the barrier is touched by anything made of starsilver, mithril, or platinum, the spell lasts continually until the caster ends it or the starsilver, mithril, or platinum is moved.