UTMB is the premium mountain trail 100-miler in Europe. Officially the route is 166.4K (103.4 miles) long, with 9,400 meters (30,840 feet) of climbing. However the real distance may well be shorter as it's possible to take a few shortcuts here and there. That's against the rules of course, but most of the 2300 competitors are from France and other Southern European countries where it's ok to break the rules. Hiking poles are allowed, but those who choose to use them must carry them all the way. This year there will be doping tests.
There are a few videos here.
You can follow the race live here (click Tete Course > Tour Mont Blanc to see Top Ten of UTMB).
By the way, there is another race called CCC on the same route and same day, which should not be confused with UTMB. It's 98K from Courmayeur (Italy) to Champex (Switzerland) to Chamonix (France). UTMB starts and finishes in Chamonix.