When I went Mac in 2008, I had 3 games on Windows that I've really longed to see on Mac OS X: Unreal Tournament 3, Risk II, and Tempest 2000.
In doing a search recently on Google I found a free demo of Lux Delux at Lux Delux website that's full featured with a limitation of number of games played. That's pretty generous!
I was WOW'ED! IMO, it's BETTER than Risk II. Graphics, Preferences, Options Galore, Online Play, Forums, etc....it's like I found my people! There are over 800 maps to play on ranging from a classic Risk board to the nether-regions of middle earth all the way to the Enterprise. the program designer is active in the forums and I like that because
If you're looking for that Risk game replacement on the Mac(there are also versions for Windows and Linux), then Lux Delux deserves a try. I've not found anything else like it that runs on the native OS X platform.