Risk Legacy is like no other game of Risk - and few other games at all - that you've ever played. Sure, it starts off similarly, what with the whole, classic "taking over the world" dealie, but soon, a few vital differences come to light.
Like the name suggests, Risk Legacy leaves a legacy: the board and rules change over time based on what happens during each game. From the start, players each choose a faction with different rules from the other ones, and select a special ability for their own faction, with unselected abilities getting completely destroyed. Other permanent changes soon come into play, staying in effect for all future games to come.
And speaking of the future, Risk Legacy's rulebook has blank pages to write rule changes, sealed packages with additional rules, and cities and resources that can be added to the map.
Risk Legacy is recommended for 3-5 players ages 13 and up, and with the same group of people each time.