Diablo III: Reaper of Souls will introduce a new end-game feature named Nephalem Trials. These special portals will open to offer really cool challenge-based encounters.
These Nephalem Trials spawn randomly in the world.
Some of these challenges include waves and waves of monsters. The quickly you deal with them get rewarded with a number of resplendant chests.
There are many things yet to be announced about Diablo III: Reaper of Souls at BlizzCon -- according to Josh Mosqueira. The size of ACT V is comparable in size to ACT II.