Hammerskull

If the hammerskull moves at least 20 feet straight toward a target and then hits it with a ram attack on the same turn, the target takes an extra 5 (2d4) bludgeoning damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.

The hammerskull has advantage on Strength and Dexterity saving throws made against effects that would knock it prone.

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d4 + 3) bludgeoning damage.