Bat Within

Being hit by an attack, you transform into a swarm of bats that moves directly to an unoccupied space within 15 feet of the cast, regaining your original form upon reaching the space. You avoid 3d6 of the damage. The swarm of bats does not provoke attacks of opportunity and is not affected by rough terrain. 

At Higher Levels. '

When you cast this spell using a spell slot of 2nd level or higher, the avoided damage increases by 2d6 for each spell slot used above 1st level.