Adult Uranium Dragon

Radioactive Aura. A radioactive aura flows from the dragon. All creatures within a 50-foot emanation originating from the dragon are poisoned until they leave the emanation. If a creature stays within the emanation for 10 minutes or longer, they are poisoned until they finish a Long Rest.

Legendary Resistance (3/Day, or 4/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast Ray of Sickness (level 3 version).

Rend. Melee Attack Roll: +12 to hit, reach 10 ft. Hit: 16 (2d8 + 7) Slashing damage plus 4 (1d8) Necrotic damage.

Radioactive Breath (Recharge 5-6). Dexterity Saving Throw: DC 19, each creature in a 70-foot Cone. Failure: 66 (12d10) Necrotic damage. Success: Half damage.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 19, +12 to hit with spell attacks):

At Will: Detect Magic, Fear, Ray of Sickness (level 3 version)

1/Day Each: Contagion, Disintegrate

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Toxic Melt. Dexterity Saving Throw: DC 19, one creature the dragon can see within 120 feet. Failure: 18 (4d8) Necrotic damage, and the target takes an extra 4 (1d8) Necrotic damage from attacks until the end of the dragon's next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Virulent Beam. The dragon uses its Spellcasting to cast Ray of Sickness (level 3 version). The dragon can’t take this action again until the start of its next turn.

Maul. The dragon makes one Rend attack.