Immutable Form. Voidwyrm is immune to any spell or effect that would alter its form.

Magic Weapons. Voidwyrm's attacks are magical.

Siege Monster. Voidwyrm deals double damage to objects and structures.

Lightning Absorption. Whenever Voidwyrm is subjected to Lightning damage, it takes no damage and instead regains a number of hit points Half to the Lightning damage dealt.

Multiattack. Voidwyrm makes two attacks: one with its bite and one with its Claw. 

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage plus 4 (1d8) cold damage.

Claw. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 6) slashing damage.

Cold Breath (Recharge 5–6). Voidwyrm exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8) cold damage on a failed save, or half as much damage on a successful one.

Bodyguard.  When an ally within 5 ft. gets hit, Voidwyrm can take the hit instead