Voice mimic. The grey dragon can mimic any voice they have heard before perfectly.

Reflective Carapace. Any time the grey dragon is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 3 to 5, the grey dragon is unaffected. On a 6, the grey dragon is unaffected, and the effect is reflected back at the caster as though it originated from the grey dragon, turning the caster into the target. 

Multiattack. The young grey dragon makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 19 (2d12 + 6) piercing damage and on a critical hit the target takes an extra 3 (1d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the grey dragon 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.

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 15 (2d8 + 6) slashing damage. 

Paralyzing Breath (Recharge 5-6). The grey dragon exhales a paralyzing mist in a 30ft cone. Each creature in that area must make a DC 18 Constitution saving throw or be paralyzed 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.