Crystal Tortoise

Magic Resistance.  The crystal Tortoise has advantage on saving throws against spells and other magic effects.

Elemental Container. The crystal tortoise possesses one type of magic determined by a d6. 1- Fire, 2- Cold, 3- Lightning, 4- Poison, 5- Radiant, 6- Necrotic. The tortoise is immune to damage of a matching type and deals damage of that type.

Internal Pressure.  At half health the tortoises crystal shell begins to crack leaking elemental magic of the tortoises type into the area around it with a radius of 5 ft. Creatures that enter this radius or start their turn there make a DC 12 Constitution save, taking  4 (2d4) damage of the tortoises type on a failed save and half as much on a successful one.

Explosive Ending. When a crystal tortoise is killed it erupts in a violent explosion of magic. Creatures within 20 ft. must make a DC 15 dexterity saving throw taking 5 (1d10) Bludgeoning damage plus 10 (3d6) damage of the tortoises type on a failed save and half as much on a successful one.

Action Name. Enter the description for your action.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft. Hit: 9 (1d5 + 4)  bludgeoning damage. 

Elemental Blast (Recharge 4-6). The Tortoise exhales elemental magic in a 15 ft. cone, each creature in that area must make a DC 15 dexterity save taking 35 (10d6) damage of the tortoises type on a failed save and half as much damage on a successful one.

Magic Eater. When the Crystal Tortoise succeeds on a saving throw against a spell that deals damage, it can use its reaction to eat the spell, fully negating its effects. The tortoise gains 1d6 hp per spell level of the eaten spell (1d4 for cantrips). If the damage type is Fire, Cold, Lightning, Poison, Radiant or Necrotic the tortoises type changes to match.