Ancient Heavy Wyrm

Reflective Plating. Any successful attack against the Ancient Heavy Wyrm will reflect half the damage back to the attacker.

Sturdy Armor. When you successfully hit the Ancient Heavy Wyrm with a weapon that does non-magical damage roll a d6. The weapon shatters on a 1-5. 

Spellcasting. The Ancient Heavy Wyrm is a 4th-level spellcaster. Its innate spellcasting ability is Intelligence (spell save DC 19, +11 to hit with spell attacks). The Wyrm can innately cast the following spells, requiring no material components.

at will: magic missile, primal savagery

6/day each: cure wounds, earth tremor

4/day each: spike growth, mirror image 

Multiattack. The Ancient Heavy Wyrm makes three attacks.

Bite. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 13 (3d8 + 1) piercing damage.

Claw. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 10 (3d6 + 1) slashing damage.

Spellcasting. The Ancient Heavy Wyrm casts a spell.

Stored Power. The Wyrm can choose not to reflect any damage it takes, and can instead imbue the damage into its spells, doubling their effects.

The Wyrm can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The Wyrm regains spent legendary actions at the start of its turn.

Bite. The Wyrm makes a Bite attack

Spellcasting. The Wyrm casts a spell