Undead. The dragon need not breathe

• Cadaverous Appearance. While the zombie dragon is lying still, it is indistinguishable from a mundane decomposing corpse. It can still be detected by a detect evil and good spell, or similar magic.
• Undead Fortitude. If damage reduces the zombie dragon to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie dragon drops to 1 hit point instead.

• Multiattack. The zombie dragon makes two attacks: one with its bite, and one with its claws.
• Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 6) piercing damage. Additionally, living creatures must succeed on a DC 18 Constitution saving throw, or take 13 (3d8) necrotic damage and contract the zombie necrosis disease (p.192).
• Claws. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.
• Tail. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 15 (2d8 + 6) slashing damage.
• Rot Breath (Recharge 5-6). The zombie dragon exhales a squall of necrotic rot in a 30-foot cone. Creatures in the area must succeed on a DC 18 Constitution saving throw, taking 31 (7d8) necrotic damage on a failed save, or half as much damage on a successful one. A creature that fails its saving throw has its hit point maximum reduced by an amount equal to the necrotic damage taken. This reduction lasts until the creature finishes a short or long rest. If the creature’s hit point maximum is reduced to 0, the creature dies.