Beast of Boskil Cliff - Shalescale Wyvern

Legendary Resistance (2/day). If the wyvern fails a saving throw, it can choose to succeed instead. 

Mimicry. The wyvern can mimic animal sounds and humanoid voices. A creature that hears the sounds can tell they are imitations with a successful DC 14 Wisdom (Insight) check.

Brittle Scales. Once the wyvern reaches 75 hitpoints or less, its AC drops to 16, but it no longer provokes attacks of opportunity while moving.

Multiattack. The wyvern makes three attacks: one with its Bite or Tail and two with its Claws.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 16
(2d10 + 5) piercing damage plus 4 (1d8) acid damage.

Claw. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 5) slashing damage.

Tail. Melee Weapon Attack: +8 to hit, reach 15 ft., one target. Hit: 19 (4d6 + 5) piercing damage and if the target is a Medium or smaller creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the wyvern can't attack with its tail.

Acid Breath (Recharge 5–6). The wyvern exhales sticky, corrosive acid in a 30-foot line that is 5 feet wide. Each creature in that area must make a DC 15 Dexterity saving throw. On failed save, a creature takes 31 (7d8) acid damage immediately and 18 (4d8) acid damage at the end of its next turn. On a successful save, a creature takes half as much of the initial damage and no damage at the end of its next turn.

Coalesce Shale (3/day). The wyvern magically coalesces a layer of gravel along its scales, granting itself 20 temporary hitpoints. While it has these hitpoints, it has resistance to bludgeoning, piercing, and slashing damage dealt by weapons that aren't adamantine. These hitpoints last for 1 minute. 

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

Claw. The wyvern makes a Claw attack.

Inflict Pain. A creature grappled on the wyvern's tail must make a DC 15 Constitution saving throw. On a failed save, the bladed tail causes the creature searing pain, and the target must subtract 1d6 from any attack roll or saving throw it makes until the start of the wyvern's next turn.

Wing Attack (Costs 2 actions). The wyvern beats its wings. Each creature within 10 feet of the wyvern must make a DC 15 Dexterity saving throw or take 12 (2d6 + 5) bludgeoning damage and be knocked prone. The wyvern can then fly up to half its flying speed.