Crystal Goblin

Stress Fractures. When the crystal goblin falls beneath 20 hit points, its armor class is reduced by 2 as some of its crystals shatter. It also deals an additional 1d4 damage with its smack attack (included in the attack).

Smack. Melee Weapon Attack: +6 to hit, reach 10 ft., 1 target. Hit: 8 (1d8 + 4) bludgeoning damage plus 6 (1d4 + 4) slashing damage. If the crystal goblin has less than 20 hit points, it deals an additional 9 (2d4 + 4) slashing damage instead of the regular amount.

Quake. The crystal goblin repeatedly slams its crystal arms into the ground, causing it to shake. All creatures within 10 feet of the crystal goblin make a DC 13 Dexterity saving throw, being knocked prone and taking 7 (2d6) bludgeoning damage on a failure.