These shackles appear to be regular iron shackles but contain faintly legible runes in an unknown script. The shackles are warded against magic and cannot be targeted by spells (such as Remove Curse or Greater Restoration to remove the cursed properties), though the Strength draining effects caused by this item CAN be targeted and removed by such spells.

(Level 0) Any creature that equips this item is cursed by it, suffering the following effects:
Strength score is reduced by 5 and the target suffers from Sunlight Weakness. While in sunlight, the creature has disadvantage on attack rolls, ability checks, and saving throws.

Choosing to attune to this item removes the above effects but also causes the character to become cursed,  gaining/suffering the following benefits/drawbacks corresponding to its level.

(Level 1) CURSED - You become unwilling to part with the shackles, and gain the ability to enter a sleep-like trance. While in this trance you manifest as a Shadow in an nearby area of dim light or darkness within 5 feet of your body, using the creature's stat block for Attack rolls, as well as all Strength, Dexterity, Constitution or Charisma checks and/or Saving Throws. The Shadow may move through any areas of dim light or darkness in an unbroken path from the host body with the effects ending instantaneously if the Shadow's trail is ever illuminated by bright light or daylight.

While awake, the wearer may choose to use the Shadow's Strength Drain ability on any Medium-size or smaller creature that is incapacitated. This Strength Drain ability must be used at least once (in either form) every 24 hours or else the shackles magically repair themselves and the initial effects of the curse return. Additionally, for each subsequent day spent without using the Strength Drain ability the wearer will suffer the effects themselves.

(Level 2) This level is unlocked by fully draining the Strength of one (1) humanoid creature until death. You retain all the effects of the previous level as well as gaining resistance to Necrotic damage darkvision to a distance of 60 feet.

Additionally, once per day while in Shadow-form the wearer may choose to swap places with their original body, taking all the items being worn or carried with them. Once this ability has been used the shackles function as if they were (Level 0) and this ability cannot be used again for 24 hours.