D&Dtools1

An ice slick spell covers a solid surface with a layer of slippery ice.
Any creature entering the area or caught in it when the spell is cast must make a successful Balance check or slip, skid, and fall.
Those that succeed on the skill check can move at half speed across the surface, or can skate or glide normally.
However, those that remain in the area must each make a new skill check every round to avoid falling and be able to move.
The DM should adjust skill checks by circumstance.
For example, a creature charging down a hill that is suddenly iced has little chance to avoid the effect, but its ability to exit the affected area is almost assured (whether it wants to or not).