Accretion. The Drake has a starting value of 0 for its AC and Str mod(D&Dbeyond wont allow me to do this tho). Whenever an attack targeted on the Drake succeeds the attack roll, its AC and Str mod both increase by 2.
Multiple hits from a single weapon (e.g Flurry of Blows) increase this multiple times, but a single attack that rolls multiple times (e.g. greatsword) only increase this once.
Magic attacks follow this rule but only increase the value by 1, spells that deal damage also trigger this effect despite not being classed as attacks.
Any environmental damage does not increase this value, nor does psychic, force or radiant damage.
This trait remains until the Drake dies, chooses to disband it's armor or otherwise stops existing.

Compound growth. At certain AC values, the Drake gets bonuses as shown below:
 - At a value of 5, the Drake's legs become larger and stronger. Its speed increases by 15ft.
 - At a value of 10, the Drake grows wings and loses its front legs, gaining a flying speed equal to its walking, and can take 2 actions per turn.
 - At 15, the Drake becomes size Huge, gains an extra +50 to its max hitpoints and increases its speed by a further 15ft
 - At 20, the Drake becomes a legendary creature and can use its legendary actions
For every 5 points above 20, its speed increases by 5ft.
For every 20 points above 20, its size increases by one, it can take 1 extra action per turn and its max hitpoints increase by 50.

Claw. Melee Weapon Attack: reach (str score)ft. cone. All creatures inside must roll a DC 8 + (Drake's str mod) Con save or take 2d8+str slashing damage. Creatures take half as much on a successful save.

Bite. Melee Weapon Attack: +(str mod) to hit, reach 10 ft., single target. Hit: +5 (2d12 + Str) piercing damage. 

Shrapnel. Ranged Weapon Attack: The Drake dismantles some of its armour, reducing its AC and Str by 1d12 and dealing that much piercing damage to every creature in a 50ft sphere centered on the Drake.

Shriek. Ranged Attack: The Drake lets out a simple cry and deals 1d12 thunder damage to every creature within 10ft.

Shed. The drake reduces its armour to 0 and heals that many hitpoints. This also allows the Drake to dash and a disengage as a part of the same action.

Armor up. After being targeted by an attack until the start of the Drake's next turn, the Drake's AC increases by the amount of damage taken, and decreases by 1 afterwards.

Back up. After being targeted by an attack, the Drake can choose to move 10ft away from the attacker in a straight line without provoking opportunity attacks.

After reaching a certain size and AC, the Drake becomes legendary, gaining 2 legendary actions per turn.

Roost. The drake immediately falls unconscious in its current spot, staying hovering if it is not grounded, healing 2d12 hitpoints at the end of its turn. It heals a further 2d12 hitpoints at the start of its next turn. The drake cannot attack and roost in the same turn, but it can use other legendary actions

Dismantle. The drake removes 1d20 armour, makes a ranged weapon attack using its strength modifier, and hurls these scraps towards a creature within sight. Each scrap deals 1d8 bludgeoning damage + 1d6 piercing damage.