A not very hard black at the top and a very long flat tail.
If you want a really long, fun, black you should go for the Villaroger run from the Aiguille Rouge in Les Arcs, France. Nearly 7,000ft of vertical and, I reckon, half the length of the Sarenne. It is the most vertical on a black run, which seems like a better boast than longest. (again, though, not a very tough black - just lots of fun).
However Alp d'Huez does have some very good, very long off piste runs, far away from any pistes in their own valleys (but you need a guide).
Snowheads is back to normal now after the attack from Turkish hackers (why?). They wiped 130,000 posts but the management has now retrieved all but 127.