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

Magical Claws. The Zaxrune's attacks count as magical for the sake of overcoming resistances.

Critical Resistance(1/Day). Once a day the Zaxrune can ignore a Critical hit and count it a a normal hit.

Three Times The Teeth And Fire (1/Day). The Zaxrune has three heads, each can breath fire at the same time and it can attack three different targets at a time. If it chooses one target with all three heads the target must make a DC 25 Dextreity saving throw or take 105 (32d6) fire damage on a failure. On a success you take half.

360. Each head can see 360 degree around itself, so it gains advantage on Perception checks.

Polymorph. As any action the Zaxrune can change into a Huge snake or a three headed snake humanoid at will. When polymorphed Zaxrune's stats stay the same, but no attacks. Zaxrune is also immune to the spell polymorph.

Intimidating Glare. If the Zazruxe chooses to it can frighten anything under level 5 or less than CR 6 requiring no saves or checks.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws, or one bite per head (three targets). The Zaxrune can substitute an attack for a spell. In humanoid forme the Zaxrune take can two scimitar attacks, and one breath attack equal to (4d10).

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 19 (2d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: +17 (2d8 + 8) bludgeoning damage, plus (1d6) poison damage.

Otherworldly Scimitar (Humanoid Form Only). Melee Weapon Attack: +19 to hit, reach 5ft., one target. Hit:  22 (4d6+10) slashing damage and 12 (2d8) psychic damage.

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.

Head One (right), Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

Head Two (center), Fire Breath (Recharge 5-6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

Head Two (center), Flammable Acid (Recharge 5–6). The dragon exhales acid in a 60-foot cone. Each creature in that area must make a DC 19 Dexterity saving throw, taking (10d6) acid damage on a failed save, or half as much damage on a successful one. If the target(s) have been hit with acid and are hit with fire within 5 rounds, the target takes 3d6 fire damage for each turn the target hasn't put it out. 

Head Three (left), Fire Breath (Recharge 5–6). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

Zaxrune can take 4 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 dragon regains spent legendary actions at the start of its turn.

Any Melee Attack Action Listed (Costs 1 Action).

Wing Attack (Costs 2 Actions). Zaxrune beats his wings. Each creature within 50 feet of him must succeed on a DC 24 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. Zaxrune can then fly up to half its flying speed.

Fire/Acid Breath (Costs 4 Actions). The Zaxrune uses his fire or acid breath attack with one head.