Asmodeus, The Raging Fiend

Shapechanger. Asmodeus can use his action to polymorph into a form that resembles a medium humanoid, another large humanoid form, or his true form. Aside from his size, his statistics are the same in each humanoid form, but when he shifts to his true form, those statistics apply (See Asmodeus, Lord of Nessus).

Tunneler. Asmodeus can burrow through solid rock at half his burrow speed and leaves a 20ft diameter tunnel in his wake. 

Legendary Resistance (5/Day). If Asmodeus fails a saving throw, he can choose to succeed instead.

Magic Resistance. Asmodeus has advantage on saving throws against spells and other magical effects.

Magic Weapons. Any weapon attacks made my Asmodeus are considered magical.

Multiattack. Asmodeus attacks twice with his bite, and can use his Frightful Presence ability.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 41 (4d12 + 15) piercing damage. 

Frightful Presence. Each creature of Asmodeus’ choice within 120 feet of him and aware of him must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. These later saves have disadvantage if Asmodeus is within line of sight of the creature.

If a creature succeeds on any of these saves or the effect ends on it, the creature is immune to Asmodeus' Frightful Presence for the next 24 hours.

Tail Sting. Melee Weapon Attack: +14 to hit, reach 15ft., one target. Hit: 26 (3d10 +10), plus 2d8 acid damage. The target must succeed on a DC 15 Dexterity saving throw or be knocked prone.

Constrict. Asmodeus chooses one creature of large or smaller size within 60ft. Make an attack roll against the target. +10 to hit, reach 60ft. Hit: 38 (8d6 + 10) bludgeoning damage, and the target is contracted by the serpent. Constricted creatures are grappled and restrained, and can use an action to escape (DC 18 STR). A constricted creature must succeed on a DC 15 Strength saving throw at the start of each of its turns, taking a further 8d6 (28) bludgeoning damage on a failed save or half as much on a success. If a target is reduced to 0 hit points while constricted, it is crushed to death and killed instantly. 

Asmodeus can only constrict one creature at a time, but can still use his bite attacks and frightful presence ability while constricting a target. 

Acid Breath (Recharge 5–6). Asmodeus exhales acid in a 90-­foot line that is 10 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 66 (12d10) acid damage on a failed save, or half as much damage on a successful one.

Asmodeus can take 5 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Asmodeus regains spent legendary actions at the start of his turn. 

Attack. Asmodeus makes one Bite attack.
Writhing Menace (Costs 2 Actions). Asmodeus attempts to Constrict a target within range, provided he isn't already constricting a target. 
Expel Bile (Costs 2 Actions). Asmodeus dredges up a collection of acidic bile and launches it at a point within 100ft, the massive globule landing with a sticky splat and covering an area of a 30ft diameter. Any creature caught in that area must succeed on a DC 18 Constitution saving throw or take 12d6 (42) acid damage, or half as much on a success. Additionally, the area becomes difficult terrain and any creature starting their turn in the area takes an additional 2d6 (7) acid damage per 5 ft of movement. If a target chooses to forego their movement and stay within the bile, they must repeat the saving throw and risk taking the 12d6 acid damage once again.