Thunder Grease

Nonflammable grease covers the ground in a 20-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

• Failed save = 4d10 thunder damage and prone condition. 
• Succeed = half damage

 A creature that enters the area or ends its turn there must also succeed on that save or fall Prone and take 1d10 thunder damage.