Need to allow swivelling/rotating fitting for steel wire
Your local mountaineering/rock climbing/etc. store (Mountain Equipment Co-Op is the one I am familiar with) will care a Swivel device that allows for rotation while still supporting the loads seen with a swing.

Swivel Image http://images.mec.ca/fluid/customers/c822/4001-684/4001-684_PDP_Image/main_variation_NOC02_view_view1_350x250.jpg?20110209163550

The model pictured is good for a working load of 5kN which is about 1124lbs and a breaking strength of 23kN. They are expensive, however.