Ankheg

The monster can burrow through solid rock at half its Burrow Speed and leaves a 10-foot-diameter tunnel in its wake.

Melee Attack Roll: (), reach ft. . If the target is a Large or smaller creature, it has the Grappled condition (escape DC ).

Dexterity Saving Throw: DC , each in a --long, --wide .
Failure: damage.
Success: Half damage.