Cerberus: Hound of Hell

Legendary Resistance (3/Day). If Cerberus fails a saving throw, it can choose to succeed instead.

Keen Hearing and Smell. Cerberus has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Ageless. Cerberus cannot suffer or die old age, or be aged magically.

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

Magic Weapons. Cerberus's weapon attacks are magical.

Sentinel at the gate. Creatures hit by an opportunity attack from Cerberus have their speed reduced to 0 until the end of their current turn. 

Multiple Heads. Cerberus has three heads, each of which can be targeted uniquely, and have an AC of 26. If any one head takes 75 points of damage, it dies, and if all three heads die, Cerberus dies. Cerberus regrows all lost heads at the end of a long rest. 

Multiattack. Cerberus makes three attacks. Each attack can be a Bite, or a Special Attack. Cerberus can't use the same Head Special Action more than once per round.

• Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 29 (4d8 + 11) piercing damage plus 10 (3d6) fire damage. If the target is a Large or smaller creature, it is grappled (escape DC 18). Until this grapple ends, the target is restrained, and Cerberus can't use this head to bite another target.
• Flaming Belch (Right Head). Ranged Weapon Attack: +12 to hit, (60/120), one target. Hit: 32 (6d8 + 4) fire damage. The target must succeed on a DC 25 Constitution saving throw or catch fire. For 1 minute, or until a creature takes an action to douse the fire, the target takes 16 (3d10) fire damage at the start of each of its turns.
• Mighty Jaws (Central Head). Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 30 (5d8 + 8) piercing damage. Cerberus critically hits with this attack with a 19 or 20 on the attack roll, and creatures who must attempt a concentration check after being damaged by this attack do so at disadvantage. 
• Draining Fang (Left Head). Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 17 (3d10) piercing damage plus 17 (3d10) necrotic damage. The target must succeed on a DC 17 Wisdom saving throw or its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest.

Howl (Recharge 5-6). Cerberus unleashes a deafening howl. Each creature within 120 feet of Cerberus that can hear it must make a DC 17 Wisdom saving throw. On a failed save, a creature takes 44 (8d10) psychic damage and is frightened and deafened for 1 minute. On a successful save, a creature takes half as much damage and isn't frightened or deafened. Frightened/deafened creatures can repeat the saving throw at the end of each of its turns, ending the effects on itself on a success. Cerberus cannot use any Head Special Actions on its next turn after using this action.

Faithful Hound.  If Cerberus is within 120 feet of Hades and can see him, it may teleport to any location it can see within 30 feet of Hades. 

Watcher at the Gate.  If Cerberus is on the Plane of Hades and can see a portal that leads to a different plane of existence, it may teleport to any location it can see within 30 feet of that portal. 

Gatekeeper. Cerberus can use a reaction to prevent one or more creatures from leaving the plane of Hades. If any creature within 150 feet of Cerberus attempts to leave the plane, including by magical means or use of an existing portal, Cerberus can use his reaction to force them to make a DC23 Charisma saving throw; if they fail the save their attempt to leave the plane fails (any spell slots or material components are used/consumed as if the spell had been successful), and they are stunned until the end of Cerberus' next turn. 

He can use this reaction up to 3 times per round. 

Cerberus 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. Cerberus regains spent legendary actions at the start of its turn.

• Bite (Costs 1 Action). Cerberus makes one Bite attack.
• Move (Costs 2 Actions). Cerberus moves up to its speed without provoking opportunity attacks. 
• Hellfire (Costs 3 Actions). Cerberus' three heads exhale hellfire in a 60-foot cone. Each creature in that area must make a DC 20 Dexterity saving throw, taking 66 (12d10) fire damage on a failed save, or half as much damage on a successful one.