Sword of Separation
This item appears to be a sword (of various kind and construction), prepared for arbitrary sizes of common sword-weilding creature.
The sword is magic, and may be granted +1, +2, or +3 to hit, depending, e.g., on the fineness of its construction and material, and the skill of its preparer.
On hitting a creature or item: roll damage, but do not deduct it from the target's HP; instead, if the total damage amount would succeed against the target's AC, the creature or item "separates" in space without being severed--if two entirely decoupled parts remain, for example, they can be moved apart independently. For creatures especially, such severed parts remain "physiologically" connected and as healthy as the creature would be without separation, but that creature cannot command or will the parts to move in any particular direction (e.g., a severed leg can bend, but a head alone would unlikely have the musculature to locomote on its own). Separated objects lose their mechanical linkages and strength across the separation, as if they had cleanly been cut, but may still pass other energy (electrical, arcane, etc.) and working fluids from seam to seam, and can be rejoined flawlessly if the seams are brought together (see below).
Particularly when an item is targeted: roll damage twice, and use only the higher of the two rolls. (Id est, the damage roll "has advantage".)
The DM has some latitude with determining what gets separated and how, or what chance the attempt has at succeeding (as with vorpal blades, the blade must generally be at least as long as the cross-section diameter to sever). As usual, the player may specify a target more closely than just "a creature" (such as a creature's neck or wrist specifically), and the DM shall propose an effective modifier to the AC for that specific scenario.
If the object or creature cannot be separated (due to a low roll, improper blade length, etc.), the sword harmlessly passes through, and no damage is dealt.
Separated parts which are pressed together along their seam (in matching orientation) rejoin. Any damage to the parts is retained; in particular, deforming the object along the seam may cause a fissure on rejoining. If this happens to parts of a creature, for example, appropriate damage may be applied.
A creature cannot benefit from any sense on a separated part which doesn't have a sensory organ there (bar exceptional sensory traits, such as blindsight or tremorsense). For example, a distant separated leg may easily sense when it is touched or moved, but cannot see nor smell, and can hear only through feeling.
Damage to the separated part of a creature is applied to that creature. At the DM's discretion, severe damage may destroy the part, at which point it loses the ability to rejoin or cause further damage.
Whenever one part of a separated object is irretrievably destroyed, the seam on the other part disappears, and the object or creature continues as if a thorough slashing injury had afflicted it there.
The AC and HP are as for the progenitor weapon.
Per detect magic, the sword has an aura of transmutation, most distinctly along the cutting edge(s) of its blade(s).