Crystalline Monstrosity

Immutable Form. The Crystalline Monstrosity is immune to any spell or effect that would alter its form.

Multiattack. The Crystaline Monstrosity can make two attacks: one with its slam and one with its crystal spike. 

Slam. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 4) bludgeoning damage.

Crystal Spike. Ranged Weapon Attack: +4 to hit, range 30/120 ft., one target. Hit: 15 (3d8 + 2) piercing damage.

Crystal Shards. As a bonus action, the Crystalline Monstrosity can shed sharp crystal shards from its body in a 10-foot radius. Each creature in the area must make a DC 13 Dexterity saving throw, taking 6 (2d6) slashing damage on a failed save, or half as much damage on a successful one.

Reactive Crystal Shield. Whenever the Crystalline Monstrosity takes damage from a melee attack, it can use its reaction to create a temporary shield of crystalline energy, giving it resistance to all damage from that attack.