This appears to be a normal light crossbow, although it has a thick wrist strap attached that is of an unusual design and toughness. On command, it can fire bolts with special properties up to three times per day. The magical bolt fired counts as a bolt +2 for determination of attack rolls (and damage if appropriate). When fired, this special bolt is trailed by a pencil-thin, snaking line of faint blue light (which radiates magic fairly strongly) that connects it to the crossbow. When the bolt impacts, the head expands into a small claw that embeds itself in the target, such that the bolt grips it strongly. By the use of a second command word, the crossbow user can “reel in” the target, the thin “cord” of magical energy shrinking to drag the target to the archer.
The speed at which the target is retrieved depends on weight, friction, and distance—the maximum speed is 60' per round. The DM may need to exercise some judgement in these cases. If the target is heavier than the archer or is immovable or braced in some way, then the crossbow archer may actually be dragged to the target rather than the reverse. This can be exploited, of course, to pull a thief up to a ceiling or up a wall. The magical cord is AC -2 and takes 15 points of damage (nonmagical weapons do no damage) to destroy.