You put 200 Lesser ???s into a convergence trap to zero your purge bar.
You put 200 Unstable ???s into the trap to trigger a purge. You need to have a large enough base to attract the purge (92+ pieces) and it needs to be inside the purge window on your server. The purge should turn up and target the trap.
The next thing to figure out is where to put the trap as you get different purges on different areas of the map. So if you want Larger Elder Things in order to try and get the items from them, your going to be triggering purges around the tower.
You can get Greater ???s (converts to 50 swirling chaos each) from the Larger Elder Things and occasionally you can get Shining ???s which are worth 100 swirling chaos each.