Uranium Dragon

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Atomic. Any poison damage dealt by the Uranium Dragon ignores resistance and immunity to poison damage.

Irradiated Presence. Whenever a creature starts it's turn within 80 ft. of the dragon, it must succeed on a DC 23 Constitution saving throw or become poisoned until the creature is healed by some kind of spell. This poison bypasses immunity to the poisoned condition, but creatures that are immune to disease are unaffected by this feature. Creatures that are already poisoned by this feature must succeed on a DC 23 Constitution saving throw or take 6 (3d4) poison damage. If the target is behind a wall of force or some other form of magical protection, they don't suffer the effects of this trait.

Half Life. The Uranium Dragon is a mostly stable entity, until it starts the exhaust. It starts with an energy score of 100. Whenever it successfully uses it's breath weapon, it's energy score is reduced by 5. At 120 health intervals while the dragon is above 152 hp, (633, 513, 393, 273, 153) it's energy score is reduced by 15. For every 2 energy missing, the dragon has an extra 1 poison damage added to it's Inhale attack. For every 10 energy score missing, The dragon gains 10 more temporary hit points during it's Inhale attack, but it's Claw, Tail, and Bite attack suffer a -1 penalty to attack and damage rolls. Once it has lost a total of 20 energy score, the dragon is not able to heal or be healed by any means.

Meltdown. When the Dragon's energy score is reduced to 0, it enters the meltdown phase. During this time, it begins to glow very brightly. Over the course of 5 rounds, the dragon becomes increasingly unstable. At the start of the dragon's turn on the sixth round, it explodes, killing it instantly. This functions even if the dragon is killed in that time. Every creature within a 300 ft. radius is instantly killed, and any non-magical items worn or carried by them are destroyed. A wave of destruction rolls out for a half mile radius, akin to the spell earthquake combined with destructive wave (pure force damage for destructive wave). In addition, for the next 5 years, the area is radiated. Whenever a creature spends a set amount of time in the area, it must make the constitution saving throw noted in the Irradiated Presence feature. The intervals are based on how close they are to the center. At a half mile away from the center, it's every 12 hours. At a quarter mile away from the center, it's every 6 hours. If they are within 150 ft. of the center, it's every 6 seconds. If the target is behind a wall of force or some other form of magical protection, they don't suffer the effects of this trait. In addition, the temperature drops by 25 degrees for the same duration.

Nuclear Depletion (Mythic Trait). When the Uranium Dragon is reduced to 0 hit points, if it still has at least 20 energy, it immediately consumes any remaining energy, reducing it's energy score to zero instantly. This does not trigger Meltdown. While the mythic trait is active, at the end of all the dragon's turns, all poisoned creatures must succeed a DC 23 Constitution saving throw or take 18 (6d6) poison damage. However, while the mythic trait is active the dragon takes 20 (5d8) force damage at the start of it's turn. This damage cannot be reduced by any means. If the dragon dies, Meltdown triggers.

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 38 (4d10 + 10) piercing damage plus 20 (6d6) poison damage.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 44 (11d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 17 (6d8 + 10) bludgeoning 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 23 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.

Inhale (Recharge 6). The dragon inhales, gaining 30 temporary hit points. If it takes this action, it cannot take a bonus action this turn, or move. On it's next turn, if it still has these temporary hit points, it exhales the built up energy in a 300 ft. cone. Each creature in that area must make a DC 23 Constitution saving throw, taking 153 (44d6) poison damage on a failed save, or half as much damage on a successful one.

The dragon 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 dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 30 (7d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

If the dragon's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Nuclear Depletion. 

Depleted Fury. The dragon can make on Tail and one Claw attack.

Nuclear Wave (Costs 2 Actions). The dragon strikes the ground, and every creature within 30 ft. of the dragon must make a DC 23 dexterity saving throw or take 16 (4d8) bludgeoning damage.

Fission (Costs 3 Actions). The dragon can roll to recharge Inhale.