This appears to be a piece of rope about a cubit long with the ends dipped in tar to prevent fraying. It will detect as alteration magic.
When thrown by a thief or assassin (only) it becomes a deadly weapon against any target with a neck or similar limb or appendage needed for breathing. On a successful to-hit roll the rope will stretch up to twice its normal length and then constrict strongly, eventually causing death. Maximum range is just 3" (but there are no penalties for range). If using weapon Vs armour adjustments, use the ones for fist/open-hand.
The target is immediately "slowed" by the loss of air and bloodflow. On each following round the victim may attempt to make a save Vs death magic modified by its strength to-hit modifier and if successful the noose has been ripped off and probably discarded. However, this is only possible if the target has two limbs capable of gripping the rope and gives up any attack routine(s) for those limbs.
Each failed save reduces the target's maximum level by 1 (so a 3/4/5 F/MU/T would go to 3/4/4, and then to 3/3/3). Once zero-level is reached, the victim is "slowed greatly".
Death follows at -1. Notice that the level reduction will impact saving throws, however hit points are not reduced.
Levels lost this way are regained at the rate of one per half hour of complete rest (again, the 3/4/5 character would go from 0/0/0 to 1/1/1, to 2/2/2, 3/3/3, 3/4/4, and finally to 4/4/5 over the course of two hours of rest).
If thrown by an assassin with surprise, a successful to-hit roll allows an immediate roll on the assassination table and if successful the victim is incapacitated and collapses into helplessness immediately. They still lose one level per round but are unable to free themselves without aid.
Naturally, shape-shifters and similar magic will be able to negate this item (unless surprised by an assassin).