Amorphous. The shadow can move through a space as narrow as 1 inch wide without squeezing.

Shadow Stealth. While in dim light or darkness, the shadow can take the Hide action as a bonus action.

Shadow Boost. While in dim light or darkness, the shadow has advantage on attack rolls, ability checks, and saving throws.

Sunlight Weakness. While in sunlight, the shadow has disadvantage on attack rolls, ability checks, and saving throws.

Tethered. While in darkness, the shadow is capable of moving 150 feet away from it's owner or as far as there is still darkness. While in dim light, the shadow can move 120 feet away from it's owner. while in normal light, the shadow can move 60 feet away from it's owner. In bright light, the shadow can move 30 feet away from it's owner. 

Black Mirror. All of the Shadow's six core stats are identical to it's owner's stats, with double their walk speed. It always has a flat AC of 16.

Strength Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 9 (2d6 + 2) necrotic damage, and the target's Strength score is reduced by 1d4, increasing by 1d4 at levels 4, 8, 12, 16, & 20. The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

If a humanoid dies from this attack, Strength Drain gains a permanent 1d4 to the drain effect.

Mimic Action. Full Action. Doing as a shadow does, it copies the actions last took by it's owner. If it's an attack, roll with advantage but only deal half damage. 

Shadow Sorcery. While in dim light or darkness, the shadow can cast any 1st level spell or cantrip at the cost of zero spell slots. At levels 5, 10, 15, & 20 add an additional level to the free spell.

All bonus actions it's owner has.

All reactions it's owner has.