Lovecraft's Horrors: "Call of the Cthulhu" World.

Slick grease covers the ground in a 10-foot square centered on a point within range and turns it into difficult terrain for the duration. When the grease appears, each creature standing in its area must succeed on a dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a dexterity saving throw or fall prone.