bag of tether stones

this wonderous item contains 20 magical stones that when activated display a number between 1-4 and then shoot out magical tethers of that number. upon attunement to the bag the number of stones becomes infinite.  the tethers have a reach of 10ft and can grapple one target per tether or combined for extra strength. to escape one tether it will require a strength save DC 15 +1 for every additional tether