This ramshackle contraption seems to be scavenged materials thrown together; wood, metal and rope all bound together in a vaguely cylindrical shape. A wide sharp point is at the base and a long cord with a ring on the end dissapears inside of it. 

While you wear this leg, your walking speed becomes 30 feet, regardless of bonuses due to its weight. As an action, The harpoon foot can be fired with a range of 20ft, using your dex modifier, and dealing 3d8 piercing damage on a hit. Any extra distance not reached (i.e firing 15ft leaves 5ft left over) is applied back to the user, pushing them in the opposite direction and dealing 1d6 bludgeoning damage if any objects/walls are hit. The harpoon takes 2 bonus actions / actions to be rewound and until then the users speed is 5ft. 

When the user receives bludgeoning damage (or at DM's discretion) they roll a d100. On a 20 or lower the device fires. A DC 15 Dex saving throw is reuired to aim the leg, otherwise it fires in a random direction. If a creature is in its path, roll to hit with no modifier if not aimed. 

Notes: Set: Innate Speed (Walking), Buff, Movement, Utility, Footwear