Corpsewyrm Fafnir, Dragon-soul of Azrael

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

Pact of Excalibur. When Fafnir reaches 0 HP, roll 1d4. Fafnir vanishes, and after rounds pass equal to the roll on the die, Arthrel (Deadking Arthrel stat block) can elect to give up half his remaining hit points on its turn to resurrect Fafnir adjacent to him, with its new hit point pool equal to the hit points Arthrel sacrificed. This requires no actions from Arthrel. If Arthrel is destroyed before the resurrection can be performed, Fafnir and Arthrel are successfully defeated.

Devour. When Fafnir deals any form of damage, it may elect to regain 10 hit points if it has at least 1 hit point and isn't in sunlight. Once this trait has been used, it cannot do so again until the start of its next turn. If Fafnir takes radiant damage or damage from holy water, this trait doesn't function at the start of Fafnir's next turn.

Joint-Soul Weaknesses. Fafnir has the following flaws:

Untethered Degradation. If Fafnir starts their turn more than 30 ft. from Arthrel, it takes 6d10 force damage.

Multiattack. The shade makes two attacks, only one of which can be an Absorb attack.

Ethereal Claws. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: (2d10 + 3) slashing damage. Instead of dealing damage, the shade can grapple the target (escape DC 17, Strength Saving Throw).

Soulfire Tail. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: (2d10 + 3) bludgeoning damage. On hit, the target must make a DC 17 Wisdom Saving Throw of fall under the effects of the Bane spell until the end of their next turn.

Absorb Essence. Melee Weapon Attack: +10 to hit, reach 5 ft., one willing creature, or a creature that is grappled by Fafnir, incapacitated, or restrained. Hit: (1d6 + 3) piercing damage plus 2d6 necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and Fafnir regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Frightful Presence: Each creature of the dragon's choice that is within 120 feet of the Dragon and aware of it must succeed on a DC 17 Wisdom saving throw or become Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, Ending the Effect on itself on a success. If a creature's saving throw is successful or the Effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Vampiric Breath (Recharge 6). The dragon creates a dark bead of necrotic force in its mouth, and then releases the energy in a 60-foot cone. Each creature in that area must make a DC 17 Charisma saving throw. On a failed save, the creature takes 6d8 necrotic damage, and Fafnir heals half of the damage dealt. On a successful save, the creature takes half as much damage, and Fafnir does not heal from any damage dealt to that creature.

Invert Natural Law. The Dragon Fafnir commands reality to invert its natural law upon seeing a healing spell or effect be cast. The caster must make a DC 17 Wisdom Saving throw. On a failure, instead of healing the amount, the spell or effect deals Psychic damage to the target instead equal to the amount that would have been healed. This effect does not work if the target to the healing spell is unconscious, or on healing potions.

Fafnir can take 3 legendary actions, choosing from the options below. They can take only one legendary action at a time and only at the end of another creature’s turn. Fafnir regains spent legendary actions at the start of their turn.

Move (Cost 1 Action). Fafnir moves up to their speed without provoking opportunity attacks.

Fatebane Gaze (Costs 2 Actions). The dragon targets a creature it can see within 60 feet of it. That target must make a DC 17 Wisdom saving throw, or have 1d4 subtracted from its Armor Class until the end of its next turn. 

Volatile Gaze (Costs 2 Actions). The dragon targets a creature it can see within 60 feet of it. That target must make an DC 17 Intelligence saving throw. On a failure, if that target takes any damage before the end of the next damage, they take an additional 2d4 force damage.

Lethargy Gaze (Costs 1 Action). The dragon targets a creature it can see within 60 feet of it. That target must make a DC 17 Constitution saving throw, or have its movement speed reduced to 0 until the end of their next turn.

Resolved Attack (Costs 1 Action). The dragon may attack a creature within range using either its Soulfire Tail or its Ethereal Claws.