Miasmorne

If a creature takes acid damage from the miasmorne and that creature is wearing nonmagical metal armor, that armor is destroyed.

After a nonmagical metal weapon hits the miasmorne that weapon melts and is destroyed. Likewise, nonmagical metal ammunition that hits the miasmorne is destroyed.

The miasmorne makes three attacks using Bite, Flechette or a combination of the two.

Melee Attack Roll: +11 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) bludgeoning damage plus 7 (2d6) acid damage. If the target is a Large or smaller creature, it has the grappled condition (escape DC 18). The miasmorne can't make Bite attacks while a creature is grappled in this way.

Ranged Attack Roll: +11 to hit, range 60/120 ft., one target. Hit: 16 (3d6 + 6) acid damage.

The miasmorne launches flechettes from its fins in a 90-foot cone in front of it. All creatures in the cone must make a DC 19 Dexterity saving throw, taking 21 (6d6) acid damage on a failed save, or half as much damage on a successful one.

The miasmorne secretes an acidic solution, then sprays it around itself. All creatures in a 20-foot-radius sphere, centered on the miasmorne, must make a DC 19 Dexterity saving throw. Targets take 26 (4d12) acid damage on a failed save, or half as much damage on a successful one. All nonmagical metals within the radius that aren't being carried, as well as nonmagical metal items carried or worn by creatures that failed the save, are destroyed.