Crypt Guard

 Undead Fortitude.  If damage reduces the Crypt Guard to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the Crypt Guard drops to 1 hit point instead.

Regeneration. The Crypt Guard regains 30 hit points at the start of its turn if it has at least 1 hit point.

Immutable Form. The Crypt Guard is immune to spells or effects that would alter its form.

Turn Immunity. The Crypt Guard is immune to the effects that turn undead.

Multiattack. The Crypt Guard makes three attacks with its Grave Sword or two uses of its Cloud of Bore Flies.

Grave Sword. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 19 (4d6 + 7) slashing damage and an additional 8 (2d6) necrotic damage.

Cloud of Bore Flies. The Crypt Guard releases a swarm of black flies that bore into the armour and skin of all creatures. The cloud is 10 ft. in radius. Every creature that starts its turn within the Cloud of Bore Flies must make a DC 16 Constitution saving throw or take 15 (4d4+7) piercing damage and have their AC reduced by 1 as the Bore Flies eat into their armour. The Cloud of Bore Flies are resistant to bludgeoning, slashing and piercing damage but are vulnerable to fire damage. The Cloud of Bore Flies has 35 hp and has an AC of 12. When the Cloud of Bore Flies is reduced to 0 hp, they explode in a poisonous dust and all creatures within the Cloud of Bore Flies must make a DC 16 Constitution Saving throw or take 12 (6d4) poison damage.