The dragon chooses a 10-foot-square area on the ground that it can see within 120 feet of it. The ground in that area turns into 3-foot-deep mud. Each creature on the ground in that area when the mud appears must succeed on a DC 15 Dexterity saving throw or sink into the mud and become restrained. A creature can take an action to attempt a DC 15 Strength check, freeing itself or another creature within its reach and ending the restrained condition on a success. Moving 1 foot in the mud costs 2 feet of movement. On initiative count 20 on the next round, the mud hardens, and the Strength DC to work free increases to 20.